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/ProductVariant.cs

@@ -0,0 +1,20 @@
+using System.Collections.Generic;
+
+namespace SkyArtShop.Models;
+
+public class ProductVariant
+{
+	public string ColorName { get; set; } = string.Empty;
+
+	public string ColorHex { get; set; } = string.Empty;
+
+	public List<string> Images { get; set; } = new List<string>();
+
+	public int StockQuantity { get; set; }
+
+	public decimal? PriceAdjustment { get; set; }
+
+	public bool IsAvailable { get; set; } = true;
+
+	public string SKU { get; set; } = string.Empty;
+}