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

Models/CollectionItem.cs

@@ -0,0 +1,10 @@
+namespace SkyArtShop.Models;
+
+public class CollectionItem
+{
+	public string Title { get; set; } = string.Empty;
+
+	public string ImageUrl { get; set; } = string.Empty;
+
+	public string Link { get; set; } = string.Empty;
+}