Add variant collections system and enhance ProductVariant with weight/stock tracking

This commit introduces a comprehensive variant management system and enhances
the existing ProductVariant model with per-variant weight overrides and stock
tracking, integrated across Admin Panel and TeleBot.

Features Added:
- Variant Collections: Reusable variant templates (e.g., "Standard Sizes")
- Admin UI for managing variant collections (CRUD operations)
- Dynamic variant editor with JavaScript-based UI
- Per-variant weight and weight unit overrides
- Per-variant stock level tracking
- SalesLedger model for financial tracking

ProductVariant Enhancements:
- Added Weight (decimal, nullable) field for variant-specific weights
- Added WeightUnit (enum, nullable) field for variant-specific units
- Maintains backward compatibility with product-level weights

TeleBot Integration:
- Enhanced variant selection UI to display stock levels
- Shows weight information with proper unit conversion (µg, g, oz, lb, ml, L)
- Compact button format: "Medium (15 in stock, 350g)"
- Real-time stock availability display

Database Migrations:
- 20250928014850_AddVariantCollectionsAndSalesLedger
- 20250928155814_AddWeightToProductVariants

Technical Changes:
- Updated Product model to support VariantCollectionId and VariantsJson
- Extended ProductService with variant collection operations
- Enhanced OrderService to handle variant-specific pricing and weights
- Updated LittleShop.Client DTOs to match server models
- Added JavaScript dynamic variant form builder

Files Modified: 15
Files Added: 17
Lines Changed: ~2000

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

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

LittleShop/Services/OrderService.cs

@@ -210,6 +210,9 @@ public class OrderService : IOrderService
     {
         var order = await _context.Orders
             .Include(o => o.Customer)
+            .Include(o => o.Items)
+            .ThenInclude(oi => oi.Product)
+            .Include(o => o.Payments)
             .FirstOrDefaultAsync(o => o.Id == id);
         if (order == null) return false;
 
@@ -231,6 +234,12 @@ public class OrderService : IOrderService
             order.ShippedAt = DateTime.UtcNow;
         }
 
+        if (updateOrderStatusDto.Status == OrderStatus.PaymentReceived && previousStatus != OrderStatus.PaymentReceived)
+        {
+            await RecordSalesLedgerAsync(order);
+            await DeductStockAsync(order);
+        }
+
         order.UpdatedAt = DateTime.UtcNow;
 
         await _context.SaveChangesAsync();
@@ -611,4 +620,52 @@ public class OrderService : IOrderService
             _ => $"Order #{orderId} status updated from {previousStatus} to {newStatus}."
         };
     }
+
+    private async Task RecordSalesLedgerAsync(Order order)
+    {
+        var payment = order.Payments.FirstOrDefault(p => p.Status == PaymentStatus.Completed);
+
+        foreach (var item in order.Items)
+        {
+            var ledgerEntry = new SalesLedger
+            {
+                Id = Guid.NewGuid(),
+                OrderId = order.Id,
+                ProductId = item.ProductId,
+                ProductName = item.Product.Name,
+                Quantity = item.Quantity,
+                SalePriceFiat = item.TotalPrice,
+                FiatCurrency = "GBP",
+                SalePriceBTC = payment?.PaidAmount,
+                Cryptocurrency = payment?.Currency.ToString(),
+                SoldAt = DateTime.UtcNow
+            };
+
+            _context.SalesLedgers.Add(ledgerEntry);
+
+            _logger.LogInformation("Recorded sales ledger entry for Order {OrderId}, Product {ProductId}, Quantity {Quantity}",
+                order.Id, item.ProductId, item.Quantity);
+        }
+    }
+
+    private async Task DeductStockAsync(Order order)
+    {
+        foreach (var item in order.Items)
+        {
+            var product = await _context.Products.FindAsync(item.ProductId);
+            if (product != null && product.StockQuantity >= item.Quantity)
+            {
+                product.StockQuantity -= item.Quantity;
+                product.UpdatedAt = DateTime.UtcNow;
+
+                _logger.LogInformation("Deducted {Quantity} units from product {ProductId} stock (Order {OrderId})",
+                    item.Quantity, item.ProductId, order.Id);
+            }
+            else if (product != null)
+            {
+                _logger.LogWarning("Insufficient stock for product {ProductId}. Order {OrderId} requires {Required} but only {Available} available",
+                    item.ProductId, order.Id, item.Quantity, product.StockQuantity);
+            }
+        }
+    }
 }