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/ShiftConfiguration.cs

@@ -0,0 +1,67 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+using WorkClub.Domain.Entities;
+
+namespace WorkClub.Infrastructure.Data.Configurations;
+
+public class ShiftConfiguration : IEntityTypeConfiguration<Shift>
+{
+    public void Configure(EntityTypeBuilder<Shift> builder)
+    {
+        builder.ToTable("shifts");
+
+        builder.HasKey(s => s.Id);
+
+        builder.Property(s => s.TenantId)
+            .IsRequired()
+            .HasMaxLength(200);
+
+        builder.Property(s => s.Title)
+            .IsRequired()
+            .HasMaxLength(200);
+
+        builder.Property(s => s.Description)
+            .HasMaxLength(2000);
+
+        builder.Property(s => s.Location)
+            .HasMaxLength(500);
+
+        builder.Property(s => s.StartTime)
+            .IsRequired();
+
+        builder.Property(s => s.EndTime)
+            .IsRequired();
+
+        builder.Property(s => s.Capacity)
+            .IsRequired()
+            .HasDefaultValue(1);
+
+        builder.Property(s => s.ClubId)
+            .IsRequired();
+
+        builder.Property(s => s.CreatedById)
+            .IsRequired();
+
+        builder.Property(s => s.CreatedAt)
+            .IsRequired();
+
+        builder.Property(s => s.UpdatedAt)
+            .IsRequired();
+
+        builder.Property(s => s.RowVersion)
+            .IsRowVersion()
+            .HasColumnName("xmin")
+            .HasColumnType("xid")
+            .ValueGeneratedOnAddOrUpdate();
+
+        builder.HasIndex(s => s.TenantId)
+            .HasDatabaseName("ix_shifts_tenant_id");
+
+        builder.HasIndex(s => s.ClubId)
+            .HasDatabaseName("ix_shifts_club_id");
+
+        builder.HasIndex(s => s.StartTime)
+            .HasDatabaseName("ix_shifts_start_time");
+    }
+}