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/BotActivity.cs

@@ -0,0 +1,56 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace LittleShop.Models;
+
+public class BotActivity
+{
+    [Key]
+    public Guid Id { get; set; }
+
+    public Guid BotId { get; set; }
+
+    [StringLength(256)]
+    public string SessionIdentifier { get; set; } = string.Empty; // Hashed user ID
+
+    [StringLength(100)]
+    public string UserDisplayName { get; set; } = string.Empty; // e.g., "Merlin", "Anonymous User #123"
+
+    [Required]
+    [StringLength(50)]
+    public string ActivityType { get; set; } = string.Empty; // e.g., "ViewProduct", "AddToCart", "Checkout", "Browse"
+
+    [StringLength(500)]
+    public string ActivityDescription { get; set; } = string.Empty; // e.g., "Viewing Red Widget", "Added 3x Blue Gadget to cart"
+
+    public Guid? ProductId { get; set; } // Related product if applicable
+
+    [StringLength(200)]
+    public string ProductName { get; set; } = string.Empty; // Denormalized for performance
+
+    public Guid? OrderId { get; set; } // Related order if applicable
+
+    [StringLength(100)]
+    public string CategoryName { get; set; } = string.Empty; // If browsing categories
+
+    public decimal? Value { get; set; } // Monetary value if applicable (cart total, order amount)
+
+    public int? Quantity { get; set; } // Quantity if applicable
+
+    [StringLength(100)]
+    public string Platform { get; set; } = "Telegram"; // Telegram, Discord, Web, etc.
+
+    [StringLength(50)]
+    public string DeviceType { get; set; } = string.Empty; // Mobile, Desktop, etc.
+
+    [StringLength(100)]
+    public string Location { get; set; } = string.Empty; // Country or region if available
+
+    public DateTime Timestamp { get; set; } = DateTime.UtcNow;
+
+    public string Metadata { get; set; } = "{}"; // JSON for additional flexible data
+
+    // Navigation properties
+    public virtual Bot Bot { get; set; } = null!;
+    public virtual Product? Product { get; set; }
+    public virtual Order? Order { get; set; }
+}