Add design approval workflow infrastructure: PATCH endpoint, email notifications, and message types

- Add PATCH method to /api/designs/[id]/route.ts for status updates
- Create sendDesignReadyForReview email template for customer notifications
- Enhance /api/designs/[id]/messages to support messageType field (MESSAGE | CHANGE_REQUEST)
- Add database migration for messageType field with MESSAGE default

This completes the backend infrastructure for the multi-step design approval workflow:
1. Admin edits and sends design to customer for review
2. Customer approves design or requests changes
3. Admin receives notifications for change requests

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

prisma/schema.prisma

@@ -196,7 +196,7 @@ model DesignApproval {
   designImageDimensions String?  // JSON array of {imageId, widthCm, heightCm} for each uploaded image
   color                String
   size                 String?
-  status               String   @default("PENDING") // PENDING | APPROVED | REJECTED
+  status               String   @default("PENDING") // PENDING | IN_REVIEW | APPROVED | REJECTED
   messages             DesignApprovalMessage[]
   createdAt            DateTime @default(now())
   updatedAt            DateTime @updatedAt
@@ -209,6 +209,7 @@ model DesignApprovalMessage {
   design       DesignApproval @relation(fields: [designId], references: [id], onDelete: Cascade)
   sender       String         // "ADMIN" | "CUSTOMER"
   body         String
+  messageType  String         @default("MESSAGE") // MESSAGE | CHANGE_REQUEST
   createdAt    DateTime       @default(now())
 }