Feature: Add product variant price override support

Enables individual variants to have their own prices, overriding the base product price.

**Database Changes:**
- Added Price (decimal?, nullable) to ProductVariants table
- Added ProductVariantId to OrderItems table with foreign key relationship
- Created index on OrderItems.ProductVariantId for performance

**API Changes:**
- ProductVariantDto: Added Price field
- CreateProductVariantDto: Added Price field with validation
- UpdateProductVariantDto: Added Price field
- OrderItemDto: Added ProductVariantId and ProductVariantName
- CreateOrderItemDto: Added ProductVariantId

**Business Logic:**
- OrderService: Variant price overrides base price (but multi-buy takes precedence)
- ProductService: All variant CRUD operations support Price field

**Admin UI:**
- CreateVariant: Price input with £ symbol and base price placeholder
- EditVariant: Price editing with £ symbol
- ProductVariants list: Shows variant price or "(base)" indicator

**Client Library:**
- Updated all DTOs to match server-side changes
- Full support for variant pricing in order creation

**Migration:**
- EF Core migration: 20251003173458_AddVariantPricing
- Backward compatible: NULL values supported for existing data

**Use Case:**
Products with size/color variants can now have different prices:
- Small T-shirt: £15.00 (variant override)
- Medium T-shirt: £18.00 (uses base price)
- Large T-shirt: £20.00 (variant override)

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

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

LittleShop/Services/OrderService.cs

@@ -78,6 +78,8 @@ public class OrderService : IOrderService
             .ThenInclude(oi => oi.Product)
             .Include(o => o.Items)
             .ThenInclude(oi => oi.ProductMultiBuy)
+            .Include(o => o.Items)
+            .ThenInclude(oi => oi.ProductVariant)
             .Include(o => o.Payments)
             .FirstOrDefaultAsync(o => o.Id == id);
 
@@ -147,8 +149,25 @@ public class OrderService : IOrderService
                 }
 
                 ProductMultiBuy? multiBuy = null;
+                ProductVariant? variant = null;
                 decimal unitPrice = product.Price;
 
+                // Check for variant price override (highest priority after multi-buy)
+                if (itemDto.ProductVariantId.HasValue)
+                {
+                    variant = await _context.ProductVariants.FindAsync(itemDto.ProductVariantId.Value);
+                    if (variant == null || !variant.IsActive || variant.ProductId != itemDto.ProductId)
+                    {
+                        throw new ArgumentException($"Product variant {itemDto.ProductVariantId} not found, inactive, or doesn't belong to product {itemDto.ProductId}");
+                    }
+
+                    // Use variant price if it has an override, otherwise use product base price
+                    if (variant.Price.HasValue)
+                    {
+                        unitPrice = variant.Price.Value;
+                    }
+                }
+
                 if (itemDto.ProductMultiBuyId.HasValue)
                 {
                     multiBuy = await _context.ProductMultiBuys.FindAsync(itemDto.ProductMultiBuyId.Value);
@@ -168,6 +187,7 @@ public class OrderService : IOrderService
                     OrderId = order.Id,
                     ProductId = itemDto.ProductId,
                     ProductMultiBuyId = itemDto.ProductMultiBuyId,
+                    ProductVariantId = itemDto.ProductVariantId,
                     Quantity = itemDto.Quantity,
                     UnitPrice = unitPrice,
                     TotalPrice = unitPrice * itemDto.Quantity
@@ -331,8 +351,10 @@ public class OrderService : IOrderService
                 Id = oi.Id,
                 ProductId = oi.ProductId,
                 ProductMultiBuyId = oi.ProductMultiBuyId,
+                ProductVariantId = oi.ProductVariantId,
                 ProductName = oi.Product.Name,
                 ProductMultiBuyName = oi.ProductMultiBuy?.Name,
+                ProductVariantName = oi.ProductVariant?.Name,
                 Quantity = oi.Quantity,
                 UnitPrice = oi.UnitPrice,
                 TotalPrice = oi.TotalPrice

LittleShop/Services/ProductService.cs

@@ -556,6 +556,7 @@ public class ProductService : IProductService
             VariantType = createVariantDto.VariantType,
             SortOrder = createVariantDto.SortOrder,
             StockLevel = createVariantDto.StockLevel,
+            Price = createVariantDto.Price,
             IsActive = true,
             CreatedAt = DateTime.UtcNow,
             UpdatedAt = DateTime.UtcNow
@@ -572,6 +573,7 @@ public class ProductService : IProductService
             VariantType = variant.VariantType,
             SortOrder = variant.SortOrder,
             StockLevel = variant.StockLevel,
+            Price = variant.Price,
             IsActive = variant.IsActive,
             CreatedAt = variant.CreatedAt,
             UpdatedAt = variant.UpdatedAt
@@ -595,6 +597,9 @@ public class ProductService : IProductService
         if (updateVariantDto.StockLevel.HasValue)
             variant.StockLevel = updateVariantDto.StockLevel.Value;
 
+        if (updateVariantDto.Price.HasValue)
+            variant.Price = updateVariantDto.Price.Value;
+
         if (updateVariantDto.IsActive.HasValue)
             variant.IsActive = updateVariantDto.IsActive.Value;
 
@@ -627,6 +632,7 @@ public class ProductService : IProductService
                 VariantType = v.VariantType,
                 SortOrder = v.SortOrder,
                 StockLevel = v.StockLevel,
+                Price = v.Price,
                 IsActive = v.IsActive,
                 CreatedAt = v.CreatedAt,
                 UpdatedAt = v.UpdatedAt
@@ -647,6 +653,7 @@ public class ProductService : IProductService
             VariantType = variant.VariantType,
             SortOrder = variant.SortOrder,
             StockLevel = variant.StockLevel,
+            Price = variant.Price,
             IsActive = variant.IsActive,
             CreatedAt = variant.CreatedAt,
             UpdatedAt = variant.UpdatedAt