fix(deps): pin gunicorn to 23.0.0

Newer gunicorn patch releases added _get_control_socket_path() which
crashes with TypeError when control_socket is None. Pin to the working
version until the gunicorn config fix propagates everywhere.

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

requirements.txt

@@ -15,7 +15,7 @@ email-validator
 et-xmlfile
 fastapi>=0.115.5
 # greenlet
-gunicorn
+gunicorn==23.0.0
 h11
 html2text
 httpcore