feat: add messaging system with admin bulk messages and user conversations

- Add conversations and direct_messages tables to schema
- User messaging page at /messages (users can start conversations, see admin responses)
- Admin messaging page at /admin/messages (view all conversations, reply to users)
- Admin bulk messaging: send to all customers, specific users, or admins
- Mail icon in navbar for logged-in users (links to appropriate messaging page)
- Show first name of admin who responded in conversation view
- Clean build cache after middleware changes

src/app/api/conversations/[id]/messages/route.ts

@@ -0,0 +1,69 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { db } from '@/lib/db';
+import { requireAuth } from '@/lib/auth';
+import { getErrorMessage } from '@/lib/errors';
+
+// Send a message to a conversation
+export async function POST(
+  request: NextRequest,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  try {
+    const user = await requireAuth();
+    if (!user) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+    }
+
+    const { id } = await params;
+    const { content } = await request.json();
+
+    if (!content || content.trim().length === 0) {
+      return NextResponse.json({ error: 'Message content is required' }, { status: 400 });
+    }
+
+    // Verify user is participant
+    const { rows: participants } = await db.query(
+      'SELECT * FROM conversation_participants WHERE conversation_id = $1 AND user_id = $2',
+      [id, user.id]
+    );
+
+    if (participants.length === 0) {
+      return NextResponse.json({ error: 'Not authorized for this conversation' }, { status: 403 });
+    }
+
+    // Create message
+    const { rows: msgRows } = await db.query(
+      'INSERT INTO direct_messages (conversation_id, sender_id, content) VALUES ($1, $2, $3) RETURNING *',
+      [id, user.id, content]
+    );
+
+    // Update conversation updated_at
+    await db.query(
+      'UPDATE conversations SET updated_at = NOW() WHERE id = $1',
+      [id]
+    );
+
+    // Get sender info
+    const { rows: userRows } = await db.query(
+      'SELECT id, username, first_name, last_name, role FROM users WHERE id = $1',
+      [user.id]
+    );
+
+    return NextResponse.json({
+      message: {
+        ...msgRows[0],
+        sender_id: user.id,
+        username: userRows[0].username,
+        first_name: userRows[0].first_name,
+        last_name: userRows[0].last_name,
+        role: userRows[0].role
+      }
+    });
+  } catch (error) {
+    console.error('Send message error:', error);
+    return NextResponse.json(
+      { error: getErrorMessage(error, 'Failed to send message') },
+      { status: 500 }
+    );
+  }
+}