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/WorkClub.Infrastructure.csproj

@@ -2,9 +2,16 @@
 
   <ItemGroup>
     <ProjectReference Include="..\WorkClub.Domain\WorkClub.Domain.csproj" />
+    <ProjectReference Include="..\WorkClub.Application\WorkClub.Application.csproj" />
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="Finbuckle.MultiTenant" Version="10.0.3" />
+    <PackageReference Include="Finbuckle.MultiTenant.AspNetCore" Version="10.0.3" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="10.0.3">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
     <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" Version="10.0.0" />
   </ItemGroup>
 
@@ -14,4 +21,8 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
+  <ItemGroup>
+    <FrameworkReference Include="Microsoft.AspNetCore.App" />
+  </ItemGroup>
+
 </Project>