Fix admin route access and backend configuration

- Added /admin redirect to login page in nginx config
- Fixed backend server.js route ordering for proper admin handling
- Updated authentication middleware and routes
- Added user management routes
- Configured PostgreSQL integration
- Updated environment configuration

Sky_Art_shop/Views/_ViewImports.cshtml

@@ -0,0 +1,3 @@
+@using SkyArtShop
+@using SkyArtShop.Models
+@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers