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]/route.ts

@@ -0,0 +1,76 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { db } from '@/lib/db';
+import { requireAuth } from '@/lib/auth';
+import { getErrorMessage } from '@/lib/errors';
+
+// Get messages for a conversation
+export async function GET(
+  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;
+
+    // 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 });
+    }
+
+    // Get all messages with sender info
+    const { rows: messages } = await db.query(`
+      SELECT 
+        dm.id,
+        dm.content,
+        dm.created_at,
+        u.id as sender_id,
+        u.username,
+        u.first_name,
+        u.last_name,
+        u.role
+      FROM direct_messages dm
+      JOIN users u ON dm.sender_id = u.id
+      WHERE dm.conversation_id = $1
+      ORDER BY dm.created_at ASC
+    `, [id]);
+
+    // Get conversation details with participants
+    const { rows: convRows } = await db.query(`
+      SELECT 
+        c.id,
+        c.subject,
+        c.created_at,
+        c.updated_at
+      FROM conversations c
+      WHERE c.id = $1
+    `, [id]);
+
+    const { rows: participantRows } = await db.query(`
+      SELECT u.id, u.username, u.first_name, u.last_name, u.role
+      FROM conversation_participants cp
+      JOIN users u ON cp.user_id = u.id
+      WHERE cp.conversation_id = $1
+    `, [id]);
+
+    return NextResponse.json({
+      conversation: convRows[0],
+      participants: participantRows,
+      messages
+    });
+  } catch (error) {
+    console.error('Get messages error:', error);
+    return NextResponse.json(
+      { error: getErrorMessage(error, 'Failed to get messages') },
+      { status: 500 }
+    );
+  }
+}