doc: use only 1 gunicorn worker

[missytake]

fix #125 for now - no simultaneous requests anymore to avoid:

2025-09-15T14:37:19.433031692Z Traceback (most recent call last):
2025-09-15T14:37:19.433045423Z   File "/usr/lib/python3.12/site-packages/flask/app.py", line 1511, in wsgi_app
2025-09-15T14:37:19.433055446Z     response = self.full_dispatch_request()
2025-09-15T14:37:19.433067763Z                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2025-09-15T14:37:19.433080931Z   File "/usr/lib/python3.12/site-packages/flask/app.py", line 919, in full_dispatch_request
2025-09-15T14:37:19.433094251Z     rv = self.handle_user_exception(e)
2025-09-15T14:37:19.433104760Z          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2025-09-15T14:37:19.433114901Z   File "/usr/lib/python3.12/site-packages/flask/app.py", line 917, in full_dispatch_request
2025-09-15T14:37:19.433125649Z     rv = self.dispatch_request()
2025-09-15T14:37:19.433136094Z          ^^^^^^^^^^^^^^^^^^^^^^^
2025-09-15T14:37:19.433146729Z   File "/usr/lib/python3.12/site-packages/flask/app.py", line 902, in dispatch_request
2025-09-15T14:37:19.433157494Z     return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)  # type: ignore[no-any-return]
2025-09-15T14:37:19.433175618Z            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2025-09-15T14:37:19.433377011Z   File "/usr/lib/python3.12/site-packages/mailadm/web.py", line 30, in new_email
2025-09-15T14:37:19.433394500Z     with db.write_transaction() as conn:
2025-09-15T14:37:19.433414241Z          ^^^^^^^^^^^^^^^^^^^^^^
2025-09-15T14:37:19.433426192Z   File "/usr/lib/python3.12/contextlib.py", line 137, in __enter__
2025-09-15T14:37:19.433448386Z     return next(self.gen)
2025-09-15T14:37:19.433462246Z            ^^^^^^^^^^^^^^
2025-09-15T14:37:19.433475399Z   File "/usr/lib/python3.12/site-packages/mailadm/db.py", line 67, in write_transaction
2025-09-15T14:37:19.433489268Z     conn = self._get_connection(closing=False, write=True, transaction=True)
2025-09-15T14:37:19.433502167Z            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2025-09-15T14:37:19.433513917Z   File "/usr/lib/python3.12/site-packages/mailadm/db.py", line 46, in _get_connection
2025-09-15T14:37:19.433528846Z     sqlconn.execute("PRAGMA journal_mode=wal")
2025-09-15T14:37:19.433542098Z sqlite3.OperationalError: disk I/O error