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);