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/Data/ApplicationDbContext.cs

@@ -0,0 +1,18 @@
+using Microsoft.AspNetCore.Identity;
+using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore;
+
+namespace SkyArtShop.Data
+{
+    public class ApplicationUser : IdentityUser
+    {
+        public string DisplayName { get; set; } = string.Empty;
+    }
+
+    public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
+    {
+        public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options)
+        {
+        }
+    }
+}