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.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")