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/SmokeTests.cs

@@ -0,0 +1,18 @@
+using System.Net;
+using WorkClub.Tests.Integration.Infrastructure;
+
+namespace WorkClub.Tests.Integration;
+
+public class SmokeTests : IntegrationTestBase
+{
+    public SmokeTests(CustomWebApplicationFactory<Program> factory) : base(factory)
+    {
+    }
+
+    [Fact]
+    public async Task HealthCheck_ReturnsOk()
+    {
+        var response = await Client.GetAsync("/health/live");
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+    }
+}