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/ProductVariant.cs

@@ -0,0 +1,31 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace LittleShop.Models;
+
+public class ProductVariant
+{
+    [Key]
+    public Guid Id { get; set; }
+
+    public Guid ProductId { get; set; }
+
+    [Required]
+    [StringLength(100)]
+    public string Name { get; set; } = string.Empty; // e.g., "Red", "Blue", "Vanilla", "Chocolate"
+
+    [StringLength(50)]
+    public string VariantType { get; set; } = "Standard"; // e.g., "Color", "Flavor", "Size", "Standard"
+
+    public int SortOrder { get; set; } = 0; // For controlling display order
+
+    public bool IsActive { get; set; } = true;
+
+    public int StockLevel { get; set; } = 0; // Optional: track stock per variant
+
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public virtual Product Product { get; set; } = null!;
+}