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/Models/Product.cs

@@ -36,7 +36,9 @@ public class Product
     // Navigation properties
     public virtual Category Category { get; set; } = null!;
     public virtual ICollection<ProductPhoto> Photos { get; set; } = new List<ProductPhoto>();
-    public virtual ICollection<ProductVariation> Variations { get; set; } = new List<ProductVariation>();
+    public virtual ICollection<ProductMultiBuy> MultiBuys { get; set; } = new List<ProductMultiBuy>();
+    public virtual ICollection<ProductVariant> Variants { get; set; } = new List<ProductVariant>();
+    public virtual ICollection<BotActivity> Activities { get; set; } = new List<BotActivity>();
     public virtual ICollection<OrderItem> OrderItems { get; set; } = new List<OrderItem>();
     public virtual ICollection<Review> Reviews { get; set; } = new List<Review>();
 }