feat(backend): add PostgreSQL schema, RLS policies, and multi-tenant middleware

- Add EF Core migrations for initial schema (clubs, members, work_items, shifts, shift_signups)
- Implement RLS policies with SET LOCAL for tenant isolation
- Add Finbuckle multi-tenant middleware with ClaimStrategy + HeaderStrategy fallback
- Create TenantValidationMiddleware to enforce JWT claims match X-Tenant-Id header
- Add tenant-aware DB interceptors (SaveChangesTenantInterceptor, TenantDbConnectionInterceptor)
- Configure AppDbContext with tenant scoping and RLS support
- Add test infrastructure: CustomWebApplicationFactory, TestAuthHandler, DatabaseFixture
- Write TDD integration tests for multi-tenant isolation and RLS enforcement
- Add health check null safety for connection string

Tasks: 7 (PostgreSQL schema + migrations + RLS), 8 (Finbuckle multi-tenancy + validation), 12 (test infrastructure)

backend/WorkClub.Domain/Entities/Shift.cs

@@ -16,5 +16,5 @@ public class Shift : ITenantEntity
     public required Guid CreatedById { get; set; }
     public required DateTimeOffset CreatedAt { get; set; }
     public required DateTimeOffset UpdatedAt { get; set; }
-    public byte[]? RowVersion { get; set; }
+    public uint RowVersion { get; set; }
 }