fix(app): restore missing import re needed by qr_code route

Accidentally removed when moving group_payments_by_person to views.py;
re.match in qr_code caused a 500 on every QR request.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app.py

@@ -1,6 +1,7 @@
 import sys
 from pathlib import Path
 from datetime import datetime
+import re
 import time
 import os
 import io