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/app/api/designs/[id]/send-to-customer/route.ts

@@ -0,0 +1,48 @@
+import { NextResponse } from 'next/server';
+import { prisma } from '@/lib/prisma';
+import { getCurrentAdmin } from '@/lib/auth';
+import { sendDesignReadyForReview } from '@/lib/mail';
+
+export async function POST(
+  req: Request,
+  { params }: { params: { id: string } }
+) {
+  try {
+    const admin = await getCurrentAdmin();
+    if (!admin) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+    }
+
+    const design = await prisma.designApproval.findUnique({
+      where: { id: params.id },
+      include: { product: true },
+    });
+
+    if (!design) {
+      return NextResponse.json({ error: 'Design not found' }, { status: 404 });
+    }
+
+    // Update status to IN_REVIEW
+    const updated = await prisma.designApproval.update({
+      where: { id: params.id },
+      data: { status: 'IN_REVIEW' },
+      include: { product: true, messages: { orderBy: { createdAt: 'asc' } } },
+    });
+
+    // Send email to customer
+    await sendDesignReadyForReview({
+      customerName: design.customerName || 'Customer',
+      customerEmail: design.customerEmail,
+      productName: design.product.name,
+      reviewLink: `${process.env.NEXT_PUBLIC_SITE_URL || 'http://localhost:3000'}/designs/${params.id}/review`,
+    });
+
+    return NextResponse.json(updated);
+  } catch (err) {
+    console.error('Error sending design to customer:', err);
+    return NextResponse.json(
+      { error: 'Failed to send design' },
+      { status: 500 }
+    );
+  }
+}