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/Controllers/DevController.cs

@@ -15,13 +15,13 @@ public class DevController : ControllerBase
         _productService = productService;
     }
 
-    [HttpPost("variations")]
-    public async Task<ActionResult<ProductVariationDto>> CreateVariationForDev(CreateProductVariationDto createVariationDto)
+    [HttpPost("multibuys")]
+    public async Task<ActionResult<ProductMultiBuyDto>> CreateMultiBuyForDev(CreateProductMultiBuyDto createMultiBuyDto)
     {
         try
         {
-            var variation = await _productService.CreateProductVariationAsync(createVariationDto);
-            return CreatedAtAction("GetProductVariation", "ProductVariations", new { id = variation.Id }, variation);
+            var multiBuy = await _productService.CreateProductMultiBuyAsync(createMultiBuyDto);
+            return CreatedAtAction("GetProductMultiBuy", "ProductMultiBuys", new { id = multiBuy.Id }, multiBuy);
         }
         catch (ArgumentException ex)
         {
@@ -37,7 +37,8 @@ public class DevController : ControllerBase
             id = p.Id,
             name = p.Name,
             price = p.Price,
-            variationCount = p.Variations.Count
+            multiBuyCount = p.MultiBuys.Count,
+            variantCount = p.Variants.Count
         });
         return Ok(result);
     }