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/Hubs/ActivityHub.cs

@@ -0,0 +1,54 @@
+using Microsoft.AspNetCore.SignalR;
+using Microsoft.AspNetCore.Authorization;
+using LittleShop.Services;
+using LittleShop.DTOs;
+
+namespace LittleShop.Hubs;
+
+[Authorize(Policy = "AdminOnly")]
+public class ActivityHub : Hub
+{
+    private readonly IBotActivityService _activityService;
+    private readonly ILogger<ActivityHub> _logger;
+
+    public ActivityHub(IBotActivityService activityService, ILogger<ActivityHub> logger)
+    {
+        _activityService = activityService;
+        _logger = logger;
+    }
+
+    public override async Task OnConnectedAsync()
+    {
+        _logger.LogInformation("Admin connected to activity hub: {ConnectionId}", Context.ConnectionId);
+
+        // Send initial summary when admin connects
+        var summary = await _activityService.GetLiveActivitySummaryAsync();
+        await Clients.Caller.SendAsync("InitialSummary", summary);
+
+        await base.OnConnectedAsync();
+    }
+
+    public override Task OnDisconnectedAsync(Exception? exception)
+    {
+        _logger.LogInformation("Admin disconnected from activity hub: {ConnectionId}", Context.ConnectionId);
+        return base.OnDisconnectedAsync(exception);
+    }
+
+    public async Task GetRecentActivities(int minutesBack = 5)
+    {
+        var activities = await _activityService.GetRecentActivitiesAsync(minutesBack);
+        await Clients.Caller.SendAsync("RecentActivities", activities);
+    }
+
+    public async Task GetActivityStats(int hoursBack = 24)
+    {
+        var stats = await _activityService.GetActivityTypeStatsAsync(hoursBack);
+        await Clients.Caller.SendAsync("ActivityStats", stats);
+    }
+
+    public async Task GetSessionActivities(string sessionIdentifier)
+    {
+        var activities = await _activityService.GetActivitiesBySessionAsync(sessionIdentifier);
+        await Clients.Caller.SendAsync("SessionActivities", activities);
+    }
+}