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/Shared/_AdminAlerts.cshtml

@@ -0,0 +1,12 @@
+@{
+    var success = TempData["SuccessMessage"] as string;
+    var error = TempData["ErrorMessage"] as string;
+}
+@if (!string.IsNullOrEmpty(success))
+{
+    <div class="alert alert-success" role="alert">@success</div>
+}
+@if (!string.IsNullOrEmpty(error))
+{
+    <div class="alert alert-danger" role="alert">@error</div>
+}