Add: Diagnostic logging for variant selection in TeleBot

Added logging to diagnose why orders are created with £0 pricing:
- Log when product has variants and variant selection is shown
- Log WARNING when product has no variants and base price is used
- Helps identify if variants are missing or not being detected

Troubleshooting: Orders showing £0 despite variants having correct prices

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

TeleBot/TeleBot/Handlers/CallbackHandler.cs

@@ -599,11 +599,14 @@ namespace TeleBot.Handlers
             // If variants exist, show variant selection with quickbuy flow
             if (product.Variants?.Any() == true)
             {
+                _logger.LogInformation("Product {ProductId} has {Count} variants, showing selection", productId, product.Variants.Count);
                 await ShowVariantSelectionForQuickBuy(bot, callbackQuery.Message!, session, product, quantity);
                 return;
             }
 
-            // Add to cart with base product
+            // Add to cart with base product (ONLY if no variants)
+            _logger.LogWarning("Quick buy for product {ProductId} ({Name}) with NO variants - using base price £{Price}",
+                productId, product.Name, product.Price);
             session.Cart.AddItem(productId, product.Name, product.Price, quantity, null, null);
 
             // Track quick buy action