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/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")