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.Infrastructure/Data/AppDbContext.cs

@@ -0,0 +1,24 @@
+using Microsoft.EntityFrameworkCore;
+using WorkClub.Domain.Entities;
+
+namespace WorkClub.Infrastructure.Data;
+
+public class AppDbContext : DbContext
+{
+    public AppDbContext(DbContextOptions<AppDbContext> options) : base(options)
+    {
+    }
+
+    public DbSet<Club> Clubs => Set<Club>();
+    public DbSet<Member> Members => Set<Member>();
+    public DbSet<WorkItem> WorkItems => Set<WorkItem>();
+    public DbSet<Shift> Shifts => Set<Shift>();
+    public DbSet<ShiftSignup> ShiftSignups => Set<ShiftSignup>();
+
+    protected override void OnModelCreating(ModelBuilder modelBuilder)
+    {
+        base.OnModelCreating(modelBuilder);
+        
+        modelBuilder.ApplyConfigurationsFromAssembly(typeof(AppDbContext).Assembly);
+    }
+}