Upgrade the Club Manager instance to PostgreSQL 15+

Production is still on PostgreSQL 14. Django 6.1 refuses to connect to anything older (minimum_database_version = (15,) in the postgres backend), so #107 can't deploy until the host's Postgres is upgraded.

Newer than 15 is fine — 15 is just the floor. Nothing in our schema needs to change for this, it's the server version only.

• Check what the host is actually running (psql -c 'SHOW server_version;') before planning anything
• Upgrade it: pg_upgrade in place if it's a stock Debian/Ubuntu install, otherwise dump/restore into a fresh 15+ cluster
• Point the app at the upgraded cluster and run ./deploy/run.sh
• Verify the app: manage.py check clean, manage.py migrate --check reports nothing pending, log in, poke around the admin
• Verify computed fields: manage.py checkdata should report 0 desynced Payment.is_successful records
• Verify the surrounding services still come up: gunicorn socket/service, nginx, the Discord bot, both post office timers, and that the email queue drains

Blocks #107.