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/LittleShop.csproj

@@ -8,6 +8,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="9.0.0" />
+    <PackageReference Include="Microsoft.AspNetCore.SignalR" Version="1.1.0" />
     <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="9.0.0">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
@@ -18,6 +19,7 @@
     <PackageReference Include="FluentValidation" Version="11.11.0" />
     <PackageReference Include="FluentValidation.AspNetCore" Version="11.3.0" />
     <PackageReference Include="Microsoft.Extensions.Diagnostics.HealthChecks.EntityFrameworkCore" Version="9.0.9" />
+    <PackageReference Include="QRCoder" Version="1.6.0" />
     <PackageReference Include="Serilog.AspNetCore" Version="8.0.3" />
     <PackageReference Include="Serilog.Sinks.File" Version="6.0.0" />
     <PackageReference Include="Swashbuckle.AspNetCore" Version="7.0.0" />