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.Tests.Integration/Infrastructure/IntegrationTestBase.cs

@@ -0,0 +1,35 @@
+using System.Text.Json;
+
+namespace WorkClub.Tests.Integration.Infrastructure;
+
+public abstract class IntegrationTestBase : IClassFixture<CustomWebApplicationFactory<Program>>, IAsyncLifetime
+{
+    protected readonly HttpClient Client;
+    protected readonly CustomWebApplicationFactory<Program> Factory;
+
+    protected IntegrationTestBase(CustomWebApplicationFactory<Program> factory)
+    {
+        Factory = factory;
+        Client = factory.CreateClient();
+    }
+
+    protected void AuthenticateAs(string email, Dictionary<string, string> clubs)
+    {
+        var clubsJson = JsonSerializer.Serialize(clubs);
+        Client.DefaultRequestHeaders.Remove("X-Test-Clubs");
+        Client.DefaultRequestHeaders.Add("X-Test-Clubs", clubsJson);
+        
+        Client.DefaultRequestHeaders.Remove("X-Test-Email");
+        Client.DefaultRequestHeaders.Add("X-Test-Email", email);
+    }
+
+    protected void SetTenant(string tenantId)
+    {
+        Client.DefaultRequestHeaders.Remove("X-Tenant-Id");
+        Client.DefaultRequestHeaders.Add("X-Tenant-Id", tenantId);
+    }
+
+    public virtual Task InitializeAsync() => Task.CompletedTask;
+
+    public virtual Task DisposeAsync() => Task.CompletedTask;
+}