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>

src/lib/mail.ts

@@ -240,3 +240,38 @@ export async function sendDesignApprovedEmail({
     return { sent: false as const, reason: err instanceof Error ? err.message : 'Unknown error sending email' };
   }
 }
+
+type SendDesignReadyForReviewArgs = {
+  customerName: string;
+  customerEmail: string;
+  productName: string;
+  reviewLink: string;
+};
+
+export async function sendDesignReadyForReview({
+  customerName,
+  customerEmail,
+  productName,
+  reviewLink,
+}: SendDesignReadyForReviewArgs) {
+  if (!process.env.SMTP_HOST) {
+    return { sent: false, reason: 'SMTP not configured — set SMTP_HOST etc. in .env' as const };
+  }
+
+  const transporter = getTransporter();
+
+  const greeting = customerName ? `Hi ${customerName},` : 'Hi,';
+
+  try {
+    await transporter.sendMail({
+      from: process.env.MAIL_FROM ?? process.env.SMTP_USER,
+      to: customerEmail,
+      subject: `Your ${productName} design is ready for review`,
+      text: `${greeting}\n\nYour ${productName} design is ready for you to review and approve.\n\nReview it here:\n${reviewLink}\n\nYou can approve the design to proceed to checkout, or request changes from the admin.\n\n— Craft2Prints`,
+      html: `<p>${greeting}</p><p>Your ${productName} design is ready for you to review and approve.</p><p><a href="${reviewLink}">${reviewLink}</a></p><p>You can approve the design to proceed to checkout, or request changes from the admin.</p><p>— Craft2Prints</p>`,
+    });
+    return { sent: true as const };
+  } catch (err) {
+    return { sent: false as const, reason: err instanceof Error ? err.message : 'Unknown error sending email' };
+  }
+}