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>

LittleShop/Areas/Admin/Controllers/DashboardController.cs

@@ -35,7 +35,8 @@ public class DashboardController : Controller
         ViewData["TotalRevenue"] = orders.Where(o => o.PaidAt.HasValue).Sum(o => o.TotalAmount).ToString("F2");
 
         // Enhanced metrics
-        ViewData["TotalVariations"] = products.Sum(p => p.Variations.Count);
+        ViewData["TotalMultiBuys"] = products.Sum(p => p.MultiBuys.Count);
+        ViewData["TotalVariants"] = products.Sum(p => p.Variants.Count);
         ViewData["PendingOrders"] = orders.Count(o => o.Status == LittleShop.Enums.OrderStatus.PendingPayment);
         ViewData["ShippedOrders"] = orders.Count(o => o.Status == LittleShop.Enums.OrderStatus.Shipped);
         ViewData["TotalStock"] = products.Sum(p => p.StockQuantity);