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/Services/IBotActivityService.cs

@@ -0,0 +1,16 @@
+using LittleShop.DTOs;
+using LittleShop.Models;
+
+namespace LittleShop.Services;
+
+public interface IBotActivityService
+{
+    Task<BotActivity> LogActivityAsync(CreateBotActivityDto dto);
+    Task<List<BotActivityDto>> GetRecentActivitiesAsync(int minutesBack = 5);
+    Task<List<BotActivityDto>> GetActivitiesBySessionAsync(string sessionIdentifier);
+    Task<List<BotActivityDto>> GetActivitiesByBotAsync(Guid botId, int limit = 100);
+    Task<LiveActivitySummaryDto> GetLiveActivitySummaryAsync();
+    Task<List<BotActivityDto>> GetProductActivitiesAsync(Guid productId, int limit = 50);
+    Task<Dictionary<string, int>> GetActivityTypeStatsAsync(int hoursBack = 24);
+    Task BroadcastActivityAsync(BotActivity activity);
+}