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.Client/Models/Product.cs

@@ -12,7 +12,8 @@ public class Product
     public string? CategoryName { get; set; }
     public bool IsActive { get; set; }
     public List<ProductPhoto> Photos { get; set; } = new();
-    public List<ProductVariation> Variations { get; set; } = new();
+    public List<ProductMultiBuy> MultiBuys { get; set; } = new();
+    public List<ProductVariant> Variants { get; set; } = new();
     public DateTime CreatedAt { get; set; }
     public DateTime UpdatedAt { get; set; }
 }
@@ -25,7 +26,7 @@ public class ProductPhoto
     public int SortOrder { get; set; }
 }
 
-public class ProductVariation
+public class ProductMultiBuy
 {
     public Guid Id { get; set; }
     public Guid ProductId { get; set; }
@@ -36,4 +37,15 @@ public class ProductVariation
     public decimal PricePerUnit { get; set; }
     public int SortOrder { get; set; }
     public bool IsActive { get; set; }
+}
+
+public class ProductVariant
+{
+    public Guid Id { get; set; }
+    public Guid ProductId { get; set; }
+    public string Name { get; set; } = string.Empty;  // e.g., "Red", "Blue", "Vanilla"
+    public string VariantType { get; set; } = "Standard";  // e.g., "Color", "Flavor"
+    public int SortOrder { get; set; }
+    public int StockLevel { get; set; }
+    public bool IsActive { get; set; }
 }