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.Api/Program.cs

@@ -51,8 +51,16 @@ builder.Services.AddAuthorizationBuilder()
 builder.Services.AddDbContext<AppDbContext>(options =>
     options.UseNpgsql(builder.Configuration.GetConnectionString("DefaultConnection")));
 
-builder.Services.AddHealthChecks()
-    .AddNpgSql(builder.Configuration.GetConnectionString("DefaultConnection")!);
+var connectionString = builder.Configuration.GetConnectionString("DefaultConnection");
+if (!string.IsNullOrEmpty(connectionString))
+{
+    builder.Services.AddHealthChecks()
+        .AddNpgSql(connectionString);
+}
+else
+{
+    builder.Services.AddHealthChecks();
+}
 
 var app = builder.Build();