fix: Add pytz dependency for Flask-BabelEx

Flask-BabelEx requires pytz, but Babel 2.14+ no longer includes it as a
transitive dependency. Add pytz explicitly.

Fixes: ModuleNotFoundError: No module named 'pytz'

Author: dahagag

requirements-webapp.txt

@@ -1,5 +1,6 @@
 Babel==2.14.0  # 2.14.0+ required for Python 3.13 (removed cgi module dependency)
 Flask-BabelEx==0.9.4
+pytz  # Required by Flask-BabelEx (no longer a transitive dep of Babel 2.14+)
 Flask-Caching==2.0.1
 Flask-Cors==3.0.10
 Flask-Migrate==3.1.0