Implement product multi-buys and variants system

Major restructuring of product variations:
- Renamed ProductVariation to ProductMultiBuy for quantity-based pricing (e.g., "3 for £25")
- Added new ProductVariant model for string-based options (colors, flavors)
- Complete separation of multi-buy pricing from variant selection

Features implemented:
- Multi-buy deals with automatic price-per-unit calculation
- Product variants for colors/flavors/sizes with stock tracking
- TeleBot checkout supports both multi-buys and variant selection
- Shopping cart correctly calculates multi-buy bundle prices
- Order system tracks selected variants and multi-buy choices
- Real-time bot activity monitoring with SignalR
- Public bot directory page with QR codes for Telegram launch
- Admin dashboard shows multi-buy and variant metrics

Technical changes:
- Updated all DTOs, services, and controllers
- Fixed cart total calculation for multi-buy bundles
- Comprehensive test coverage for new functionality
- All existing tests passing with new features

Database changes:
- Migrated ProductVariations to ProductMultiBuys
- Added ProductVariants table
- Updated OrderItems to track variants

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

TeleBot/TeleBot/Models/ShoppingCart.cs

@@ -11,22 +11,30 @@ namespace TeleBot.Models
         public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
         public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
         
-        public void AddItem(Guid productId, string productName, decimal price, int quantity = 1, Guid? variationId = null)
+        public void AddItem(Guid productId, string productName, decimal price, int quantity = 1, Guid? multiBuyId = null, string? selectedVariant = null)
         {
             var existingItem = Items.FirstOrDefault(i =>
-                i.ProductId == productId && i.VariationId == variationId);
+                i.ProductId == productId && i.MultiBuyId == multiBuyId && i.SelectedVariant == selectedVariant);
 
             if (existingItem != null)
             {
-                existingItem.Quantity += quantity;
-                existingItem.UpdateTotalPrice();
+                // For multi-buys, we don't add quantities - each multi-buy is a separate bundle
+                // For regular items, we add the quantities together
+                if (!multiBuyId.HasValue)
+                {
+                    existingItem.Quantity += quantity;
+                    existingItem.UpdateTotalPrice();
+                }
+                // If it's a multi-buy and already exists, we don't add it again
+                // (user should explicitly add another multi-buy bundle if they want more)
             }
             else
             {
                 var newItem = new CartItem
                 {
                     ProductId = productId,
-                    VariationId = variationId,
+                    MultiBuyId = multiBuyId,
+                    SelectedVariant = selectedVariant,
                     ProductName = productName,
                     UnitPrice = price,
                     Quantity = quantity
@@ -87,20 +95,30 @@ namespace TeleBot.Models
     public class CartItem
     {
         public Guid ProductId { get; set; }
-        public Guid? VariationId { get; set; }
+        public Guid? MultiBuyId { get; set; }  // For quantity pricing (e.g., 3 for £25)
+        public string? SelectedVariant { get; set; }  // For color/flavor selection
         public string ProductName { get; set; } = string.Empty;
         public int Quantity { get; set; }
-        public decimal UnitPrice { get; set; }
+        public decimal UnitPrice { get; set; }  // For multi-buys, this is the bundle price; for regular items, it's per-unit
         public decimal TotalPrice { get; set; }
-        
+
         public CartItem()
         {
             // Don't calculate total in constructor - wait for properties to be set
         }
-        
+
         public void UpdateTotalPrice()
         {
-            TotalPrice = UnitPrice * Quantity;
+            // For multi-buys, UnitPrice is already the total bundle price
+            // For regular items, we need to multiply by quantity
+            if (MultiBuyId.HasValue)
+            {
+                TotalPrice = UnitPrice; // Bundle price, not multiplied
+            }
+            else
+            {
+                TotalPrice = UnitPrice * Quantity; // Regular per-unit pricing
+            }
         }
     }
 }