feat(api/offline): idempotency-key middleware for safe write retries

Backend half of offline Phase 1. Lets the offline outbox replay a write after a
lost response without executing it twice (e.g. an order whose POST reached the
server but whose reply never came back).

- IdempotencyRecord entity + table (unique index on (Scope, Key)); migration
  AddIdempotencyRecords. Standalone POCO — no tenant/soft-delete filters.
- IdempotencyMiddleware (after TenantMiddleware, before plan-limit/controllers):
  opt-in via `Idempotency-Key` header on POST/PUT/PATCH/DELETE.
    * Completed key → replays stored status+body with `Idempotent-Replay: true`.
    * In-progress key → 409 IDEMPOTENCY_IN_PROGRESS; the unique index serializes
      racing first requests; stale (>60s) reservations are recovered after a crash.
    * Only <500 responses are cached; 5xx is released so the client can retry.
  Bookkeeping runs in isolated DI scopes so it never contaminates the controller's
  unit of work. Keys are scoped per café — no cross-tenant collisions.
- 5 middleware tests (replay/execute-once, distinct key, pass-through, tenant
  isolation, 5xx-not-cached). Full suite 86 passing.

Next in Phase 1: generalize the POS order queue into a generic client outbox that
sends these keys and remaps client→server ids.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Meezi.API/Extensions/ServiceCollectionExtensions.cs

@@ -215,6 +215,9 @@ public static class ServiceCollectionExtensions
         app.UseMeeziSecurity();
         app.UseAuthentication();
         app.UseMiddleware<Middleware.TenantMiddleware>();
+        // After tenant context (keys are scoped per café), before plan-limit + controllers
+        // so a replayed write short-circuits without re-consuming limits or re-executing.
+        app.UseMiddleware<Middleware.IdempotencyMiddleware>();
         app.UseMiddleware<Middleware.PlanLimitMiddleware>();
         app.UseAuthorization();
 

src/Meezi.API/Middleware/IdempotencyMiddleware.cs

@@ -0,0 +1,188 @@
+using System.Text;
+using Microsoft.EntityFrameworkCore;
+using Meezi.Core.Entities;
+using Meezi.Core.Enums;
+using Meezi.Core.Interfaces;
+using Meezi.Infrastructure.Data;
+
+namespace Meezi.API.Middleware;
+
+/// <summary>
+/// Makes mutating requests safe to retry. A client (e.g. the offline outbox)
+/// attaches an <c>Idempotency-Key</c> header; if the same key is seen again, the
+/// original response is replayed instead of executing the write twice.
+///
+/// Bookkeeping runs in isolated DI scopes so it never mixes with the controller's
+/// own DbContext unit of work. Opt-in via header → non-idempotent and binary/file
+/// endpoints are unaffected unless the client explicitly sends a key.
+/// </summary>
+public class IdempotencyMiddleware
+{
+    private const string HeaderName = "Idempotency-Key";
+    private const int MaxKeyLength = 200;
+    private const int MaxStoredBodyBytes = 256 * 1024;
+    /// <summary>An InProgress record older than this is assumed crashed mid-flight and re-run.</summary>
+    private static readonly TimeSpan StaleInProgress = TimeSpan.FromSeconds(60);
+
+    private readonly RequestDelegate _next;
+    private readonly ILogger<IdempotencyMiddleware> _logger;
+
+    public IdempotencyMiddleware(RequestDelegate next, ILogger<IdempotencyMiddleware> logger)
+    {
+        _next = next;
+        _logger = logger;
+    }
+
+    public async Task InvokeAsync(HttpContext context, ITenantContext tenant, IServiceScopeFactory scopeFactory)
+    {
+        var method = context.Request.Method;
+        var isMutating = HttpMethods.IsPost(method) || HttpMethods.IsPut(method)
+            || HttpMethods.IsPatch(method) || HttpMethods.IsDelete(method);
+
+        if (!isMutating || !context.Request.Headers.TryGetValue(HeaderName, out var headerValues))
+        {
+            await _next(context);
+            return;
+        }
+
+        var key = headerValues.ToString();
+        if (string.IsNullOrWhiteSpace(key) || key.Length > MaxKeyLength)
+        {
+            // Unusable key — behave as if it wasn't sent rather than reject the write.
+            await _next(context);
+            return;
+        }
+
+        var scope = string.IsNullOrEmpty(tenant.CafeId) ? "global" : tenant.CafeId;
+        var path = context.Request.Path.Value ?? string.Empty;
+
+        // 1) Look for an existing record for this (tenant, key).
+        await using (var lookupScope = scopeFactory.CreateAsyncScope())
+        {
+            var db = lookupScope.ServiceProvider.GetRequiredService<AppDbContext>();
+            var existing = await db.IdempotencyRecords.AsNoTracking()
+                .FirstOrDefaultAsync(r => r.Scope == scope && r.Key == key, context.RequestAborted);
+
+            if (existing is not null)
+            {
+                if (existing.Status == IdempotencyStatus.Completed)
+                {
+                    await ReplayAsync(context, existing);
+                    return;
+                }
+                if (DateTime.UtcNow - existing.CreatedAt < StaleInProgress)
+                {
+                    await WriteConflictAsync(context); // genuine concurrent duplicate
+                    return;
+                }
+                // Stale reservation (process likely crashed mid-flight) — drop and re-run.
+                _logger.LogWarning("Recovering stale idempotency reservation {Key} for scope {Scope}", key, scope);
+                var stale = await db.IdempotencyRecords
+                    .FirstOrDefaultAsync(r => r.Id == existing.Id, context.RequestAborted);
+                if (stale is not null)
+                {
+                    db.IdempotencyRecords.Remove(stale);
+                    await db.SaveChangesAsync(context.RequestAborted);
+                }
+            }
+        }
+
+        // 2) Reserve the key. The unique (Scope, Key) index serializes racing first requests.
+        var record = new IdempotencyRecord
+        {
+            Scope = scope,
+            Key = key,
+            Method = method,
+            Path = path,
+            Status = IdempotencyStatus.InProgress,
+        };
+        try
+        {
+            await using var reserveScope = scopeFactory.CreateAsyncScope();
+            var db = reserveScope.ServiceProvider.GetRequiredService<AppDbContext>();
+            db.IdempotencyRecords.Add(record);
+            await db.SaveChangesAsync(context.RequestAborted);
+        }
+        catch (DbUpdateException)
+        {
+            await WriteConflictAsync(context); // another request won the reservation race
+            return;
+        }
+
+        // 3) Run the real request, capturing its response.
+        var originalBody = context.Response.Body;
+        await using var buffer = new MemoryStream();
+        context.Response.Body = buffer;
+        try
+        {
+            await _next(context);
+        }
+        catch
+        {
+            context.Response.Body = originalBody;
+            await DeleteAsync(scopeFactory, record.Id);
+            throw;
+        }
+
+        var statusCode = context.Response.StatusCode;
+        buffer.Position = 0;
+        var bytes = buffer.ToArray();
+        context.Response.Body = originalBody;
+        if (bytes.Length > 0)
+            await originalBody.WriteAsync(bytes, context.RequestAborted);
+
+        // 4) Persist the result so retries replay it — except 5xx, which is transient and
+        //    released so the client can retry the same key.
+        if (statusCode is >= 200 and < 500)
+        {
+            var storedBody = bytes.Length is > 0 and <= MaxStoredBodyBytes
+                ? Encoding.UTF8.GetString(bytes)
+                : null;
+            await CompleteAsync(scopeFactory, record.Id, statusCode, storedBody);
+        }
+        else
+        {
+            await DeleteAsync(scopeFactory, record.Id);
+        }
+    }
+
+    private static async Task ReplayAsync(HttpContext context, IdempotencyRecord record)
+    {
+        context.Response.StatusCode = record.ResponseStatusCode;
+        context.Response.ContentType = "application/json; charset=utf-8";
+        context.Response.Headers["Idempotent-Replay"] = "true";
+        if (!string.IsNullOrEmpty(record.ResponseBody))
+            await context.Response.WriteAsync(record.ResponseBody);
+    }
+
+    private static async Task WriteConflictAsync(HttpContext context)
+    {
+        context.Response.StatusCode = StatusCodes.Status409Conflict;
+        context.Response.ContentType = "application/json; charset=utf-8";
+        await context.Response.WriteAsync(
+            "{\"success\":false,\"data\":null,\"error\":{\"code\":\"IDEMPOTENCY_IN_PROGRESS\",\"message\":\"A request with this key is still being processed.\"}}");
+    }
+
+    private static async Task CompleteAsync(IServiceScopeFactory f, string id, int status, string? body)
+    {
+        await using var s = f.CreateAsyncScope();
+        var db = s.ServiceProvider.GetRequiredService<AppDbContext>();
+        var rec = await db.IdempotencyRecords.FirstOrDefaultAsync(r => r.Id == id);
+        if (rec is null) return;
+        rec.Status = IdempotencyStatus.Completed;
+        rec.ResponseStatusCode = status;
+        rec.ResponseBody = body;
+        rec.CompletedAt = DateTime.UtcNow;
+        await db.SaveChangesAsync();
+    }
+
+    private static async Task DeleteAsync(IServiceScopeFactory f, string id)
+    {
+        await using var s = f.CreateAsyncScope();
+        var db = s.ServiceProvider.GetRequiredService<AppDbContext>();
+        var rec = await db.IdempotencyRecords.FirstOrDefaultAsync(r => r.Id == id);
+        if (rec is null) return;
+        db.IdempotencyRecords.Remove(rec);
+        await db.SaveChangesAsync();
+    }
+}

src/Meezi.Core/Entities/IdempotencyRecord.cs

@@ -0,0 +1,31 @@
+using Meezi.Core.Enums;
+
+namespace Meezi.Core.Entities;
+
+/// <summary>
+/// Records a client-supplied Idempotency-Key so a retried write (e.g. an order
+/// replayed from the offline outbox after a lost response) returns the original
+/// result instead of executing twice. Standalone POCO — deliberately not a
+/// TenantEntity, to avoid soft-delete/tenant query filters.
+/// </summary>
+public class IdempotencyRecord
+{
+    public string Id { get; set; } = Guid.NewGuid().ToString("N");
+
+    /// <summary>Tenant scope (CafeId), or "global" for non-tenant requests.</summary>
+    public string Scope { get; set; } = "global";
+
+    /// <summary>The client-supplied Idempotency-Key header value.</summary>
+    public string Key { get; set; } = string.Empty;
+
+    public string Method { get; set; } = string.Empty;
+    public string Path { get; set; } = string.Empty;
+
+    public IdempotencyStatus Status { get; set; } = IdempotencyStatus.InProgress;
+
+    public int ResponseStatusCode { get; set; }
+    public string? ResponseBody { get; set; }
+
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime? CompletedAt { get; set; }
+}

src/Meezi.Core/Enums/IdempotencyStatus.cs

@@ -0,0 +1,9 @@
+namespace Meezi.Core.Enums;
+
+public enum IdempotencyStatus
+{
+    /// <summary>Reserved; the original request is still executing.</summary>
+    InProgress = 0,
+    /// <summary>Finished; the stored response is replayed on duplicate keys.</summary>
+    Completed = 1
+}

src/Meezi.Infrastructure/Data/AppDbContext.cs

@@ -82,10 +82,25 @@ public class AppDbContext : DbContext
     // Immutable audit trail of sensitive POS / management actions.
     public DbSet<AuditLog> AuditLogs => Set<AuditLog>();
 
+    // Idempotency keys for safe retry of offline-replayed writes.
+    public DbSet<IdempotencyRecord> IdempotencyRecords => Set<IdempotencyRecord>();
+
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
         base.OnModelCreating(modelBuilder);
 
+        modelBuilder.Entity<IdempotencyRecord>(e =>
+        {
+            e.HasKey(x => x.Id);
+            // One result per (tenant, key). The unique index also serializes
+            // concurrent first-time requests carrying the same key.
+            e.HasIndex(x => new { x.Scope, x.Key }).IsUnique();
+            e.Property(x => x.Scope).HasMaxLength(64).IsRequired();
+            e.Property(x => x.Key).HasMaxLength(200).IsRequired();
+            e.Property(x => x.Method).HasMaxLength(10).IsRequired();
+            e.Property(x => x.Path).HasMaxLength(512).IsRequired();
+        });
+
         modelBuilder.Entity<PushDevice>(e =>
         {
             e.HasKey(x => x.Id);

src/Meezi.Infrastructure/Data/Migrations/20260602142331_AddIdempotencyRecords.cs

@@ -0,0 +1,48 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Meezi.Infrastructure.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddIdempotencyRecords : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "IdempotencyRecords",
+                columns: table => new
+                {
+                    Id = table.Column<string>(type: "text", nullable: false),
+                    Scope = table.Column<string>(type: "character varying(64)", maxLength: 64, nullable: false),
+                    Key = table.Column<string>(type: "character varying(200)", maxLength: 200, nullable: false),
+                    Method = table.Column<string>(type: "character varying(10)", maxLength: 10, nullable: false),
+                    Path = table.Column<string>(type: "character varying(512)", maxLength: 512, nullable: false),
+                    Status = table.Column<int>(type: "integer", nullable: false),
+                    ResponseStatusCode = table.Column<int>(type: "integer", nullable: false),
+                    ResponseBody = table.Column<string>(type: "text", nullable: true),
+                    CreatedAt = table.Column<DateTime>(type: "timestamp with time zone", nullable: false),
+                    CompletedAt = table.Column<DateTime>(type: "timestamp with time zone", nullable: true)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_IdempotencyRecords", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_IdempotencyRecords_Scope_Key",
+                table: "IdempotencyRecords",
+                columns: new[] { "Scope", "Key" },
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "IdempotencyRecords");
+        }
+    }
+}

src/Meezi.Infrastructure/Data/Migrations/AppDbContextModelSnapshot.cs

@@ -1129,6 +1129,54 @@ namespace Meezi.Infrastructure.Data.Migrations
                     b.ToTable("Expenses");
                 });
 
+            modelBuilder.Entity("Meezi.Core.Entities.IdempotencyRecord", b =>
+                {
+                    b.Property<string>("Id")
+                        .HasColumnType("text");
+
+                    b.Property<DateTime?>("CompletedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<DateTime>("CreatedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<string>("Key")
+                        .IsRequired()
+                        .HasMaxLength(200)
+                        .HasColumnType("character varying(200)");
+
+                    b.Property<string>("Method")
+                        .IsRequired()
+                        .HasMaxLength(10)
+                        .HasColumnType("character varying(10)");
+
+                    b.Property<string>("Path")
+                        .IsRequired()
+                        .HasMaxLength(512)
+                        .HasColumnType("character varying(512)");
+
+                    b.Property<string>("ResponseBody")
+                        .HasColumnType("text");
+
+                    b.Property<int>("ResponseStatusCode")
+                        .HasColumnType("integer");
+
+                    b.Property<string>("Scope")
+                        .IsRequired()
+                        .HasMaxLength(64)
+                        .HasColumnType("character varying(64)");
+
+                    b.Property<int>("Status")
+                        .HasColumnType("integer");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Scope", "Key")
+                        .IsUnique();
+
+                    b.ToTable("IdempotencyRecords");
+                });
+
             modelBuilder.Entity("Meezi.Core.Entities.Ingredient", b =>
                 {
                     b.Property<string>("Id")