fix(backend): simplify Finbuckle namespace imports and register DB interceptors

- Use consolidated Finbuckle.MultiTenant namespace instead of separate imports
- Switch TenantProvider to use untyped IMultiTenantContextAccessor (Finbuckle 9.x pattern)
- Register TenantDbConnectionInterceptor and SaveChangesTenantInterceptor as singletons
- Add interceptors to DbContext configuration for RLS tenant context support
- Update evidence files for Task 7 and Task 8 verification

backend/WorkClub.Infrastructure/Services/TenantProvider.cs

@@ -9,11 +9,11 @@ namespace WorkClub.Infrastructure.Services;
 
 public class TenantProvider : ITenantProvider
 {
-    private readonly IMultiTenantContextAccessor<TenantInfo> _multiTenantContextAccessor;
+    private readonly IMultiTenantContextAccessor _multiTenantContextAccessor;
     private readonly IHttpContextAccessor _httpContextAccessor;
 
     public TenantProvider(
-        IMultiTenantContextAccessor<TenantInfo> multiTenantContextAccessor,
+        IMultiTenantContextAccessor multiTenantContextAccessor,
         IHttpContextAccessor httpContextAccessor)
     {
         _multiTenantContextAccessor = multiTenantContextAccessor;