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

Data/ApplicationUser.cs

@@ -0,0 +1,8 @@
+using Microsoft.AspNetCore.Identity;
+
+namespace SkyArtShop.Data;
+
+public class ApplicationUser : IdentityUser
+{
+	public string DisplayName { get; set; } = string.Empty;
+}