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/Program.cs

@@ -102,6 +102,10 @@ builder.Services.AddScoped<IPushNotificationService, PushNotificationService>();
 builder.Services.AddHttpClient<ITeleBotMessagingService, TeleBotMessagingService>();
 builder.Services.AddScoped<IProductImportService, ProductImportService>();
 builder.Services.AddSingleton<ITelegramBotManagerService, TelegramBotManagerService>();
+builder.Services.AddScoped<IBotActivityService, BotActivityService>();
+
+// SignalR
+builder.Services.AddSignalR();
 
 // Health Checks
 builder.Services.AddHealthChecks()
@@ -243,6 +247,9 @@ app.MapControllerRoute(
 
 app.MapControllers(); // API routes
 
+// Map SignalR hub
+app.MapHub<LittleShop.Hubs.ActivityHub>("/activityHub");
+
 // Health check endpoint
 app.MapHealthChecks("/health");