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/Services/IProductService.cs

@@ -15,10 +15,17 @@ public interface IProductService
     Task<bool> RemoveProductPhotoAsync(Guid productId, Guid photoId);
     Task<IEnumerable<ProductDto>> SearchProductsAsync(string searchTerm);
 
-    // Product Variations
-    Task<ProductVariationDto> CreateProductVariationAsync(CreateProductVariationDto createVariationDto);
-    Task<bool> UpdateProductVariationAsync(Guid id, UpdateProductVariationDto updateVariationDto);
-    Task<bool> DeleteProductVariationAsync(Guid id);
-    Task<IEnumerable<ProductVariationDto>> GetProductVariationsAsync(Guid productId);
-    Task<ProductVariationDto?> GetProductVariationByIdAsync(Guid id);
+    // Product Multi-Buys
+    Task<ProductMultiBuyDto> CreateProductMultiBuyAsync(CreateProductMultiBuyDto createMultiBuyDto);
+    Task<bool> UpdateProductMultiBuyAsync(Guid id, UpdateProductMultiBuyDto updateMultiBuyDto);
+    Task<bool> DeleteProductMultiBuyAsync(Guid id);
+    Task<IEnumerable<ProductMultiBuyDto>> GetProductMultiBuysAsync(Guid productId);
+    Task<ProductMultiBuyDto?> GetProductMultiBuyByIdAsync(Guid id);
+
+    // Product Variants
+    Task<ProductVariantDto> CreateProductVariantAsync(CreateProductVariantDto createVariantDto);
+    Task<bool> UpdateProductVariantAsync(Guid id, UpdateProductVariantDto updateVariantDto);
+    Task<bool> DeleteProductVariantAsync(Guid id);
+    Task<IEnumerable<ProductVariantDto>> GetProductVariantsAsync(Guid productId);
+    Task<ProductVariantDto?> GetProductVariantByIdAsync(Guid id);
 }