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/DTOs/ProductDto.cs

@@ -18,7 +18,8 @@ public class ProductDto
     public DateTime UpdatedAt { get; set; }
     public bool IsActive { get; set; }
     public List<ProductPhotoDto> Photos { get; set; } = new();
-    public List<ProductVariationDto> Variations { get; set; } = new();
+    public List<ProductMultiBuyDto> MultiBuys { get; set; } = new();
+    public List<ProductVariantDto> Variants { get; set; } = new();
 }
 
 public class ProductPhotoDto
@@ -91,7 +92,7 @@ public class CreateProductPhotoDto
     public int DisplayOrder { get; set; }
 }
 
-public class ProductVariationDto
+public class ProductMultiBuyDto
 {
     public Guid Id { get; set; }
     public Guid ProductId { get; set; }
@@ -106,7 +107,20 @@ public class ProductVariationDto
     public DateTime UpdatedAt { get; set; }
 }
 
-public class CreateProductVariationDto
+public class ProductVariantDto
+{
+    public Guid Id { get; set; }
+    public Guid ProductId { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public string VariantType { get; set; } = "Standard";
+    public int SortOrder { get; set; }
+    public int StockLevel { get; set; }
+    public bool IsActive { get; set; }
+    public DateTime CreatedAt { get; set; }
+    public DateTime UpdatedAt { get; set; }
+}
+
+public class CreateProductMultiBuyDto
 {
     [Required]
     public Guid ProductId { get; set; }
@@ -129,7 +143,26 @@ public class CreateProductVariationDto
     public int SortOrder { get; set; }
 }
 
-public class UpdateProductVariationDto
+public class CreateProductVariantDto
+{
+    [Required]
+    public Guid ProductId { get; set; }
+
+    [Required]
+    [StringLength(100)]
+    public string Name { get; set; } = string.Empty;
+
+    [StringLength(50)]
+    public string VariantType { get; set; } = "Standard";
+
+    [Range(0, int.MaxValue)]
+    public int SortOrder { get; set; }
+
+    [Range(0, int.MaxValue)]
+    public int StockLevel { get; set; } = 0;
+}
+
+public class UpdateProductMultiBuyDto
 {
     [StringLength(100)]
     public string? Name { get; set; }
@@ -145,5 +178,22 @@ public class UpdateProductVariationDto
     [Range(0, int.MaxValue)]
     public int? SortOrder { get; set; }
 
+    public bool? IsActive { get; set; }
+}
+
+public class UpdateProductVariantDto
+{
+    [StringLength(100)]
+    public string? Name { get; set; }
+
+    [StringLength(50)]
+    public string? VariantType { get; set; }
+
+    [Range(0, int.MaxValue)]
+    public int? SortOrder { get; set; }
+
+    [Range(0, int.MaxValue)]
+    public int? StockLevel { get; set; }
+
     public bool? IsActive { get; set; }
 }