fix: repeat subquery in ORDER BY instead of using alias (PostgreSQL doesn't resolve SELECT aliases inside COALESCE)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/routes/chats.ts

@@ -44,7 +44,10 @@ export default async function chatRoutes(app: FastifyInstance) {
         ) END AS private_user_id
       FROM chats c
       JOIN chat_members cm ON cm.chat_id = c.id AND cm.user_id = $1
-      ORDER BY COALESCE(last_message_at, c.created_at) DESC
+      ORDER BY COALESCE(
+        (SELECT created_at FROM messages WHERE chat_id = c.id AND is_deleted = FALSE ORDER BY created_at DESC LIMIT 1),
+        c.created_at
+      ) DESC
     `, [userId]);
 
     return rows.map(r => ({