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/Configurations/ShiftSignupConfiguration.cs

@@ -0,0 +1,38 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using WorkClub.Domain.Entities;
+
+namespace WorkClub.Infrastructure.Data.Configurations;
+
+public class ShiftSignupConfiguration : IEntityTypeConfiguration<ShiftSignup>
+{
+    public void Configure(EntityTypeBuilder<ShiftSignup> builder)
+    {
+        builder.ToTable("shift_signups");
+
+        builder.HasKey(ss => ss.Id);
+
+        builder.Property(ss => ss.TenantId)
+            .IsRequired()
+            .HasMaxLength(200);
+
+        builder.Property(ss => ss.ShiftId)
+            .IsRequired();
+
+        builder.Property(ss => ss.MemberId)
+            .IsRequired();
+
+        builder.Property(ss => ss.SignedUpAt)
+            .IsRequired();
+
+        builder.HasIndex(ss => ss.TenantId)
+            .HasDatabaseName("ix_shift_signups_tenant_id");
+
+        builder.HasIndex(ss => ss.ShiftId)
+            .HasDatabaseName("ix_shift_signups_shift_id");
+
+        builder.HasIndex(ss => new { ss.ShiftId, ss.MemberId })
+            .IsUnique()
+            .HasDatabaseName("ix_shift_signups_shift_member");
+    }
+}