Run the periodic commands in a scheduler service (no host cron)

Adds deploy/scheduler-entrypoint.sh to the backend image (entrypoint
j621-scheduler) and a scheduler service to the four composes that have a
backend. It waits until the database and migrations are ready, runs every
job once, then keeps to the intervals:

  sync_followed_tags + sync_followed_pools   every 30 min (J621_SYNC_EVERY)
  cleanup_similarity                         hourly      (J621_CLEAN_EVERY)
  refresh_guest_blacklist                    daily       (J621_BLACKLIST_EVERY)

It shares the backend image, media volume and env file, so commands see
the same library and database; failures are logged and retried next
interval. Output goes to docker compose logs scheduler; the frontend-only
composes have no backend and therefore no scheduler.

Verified against a real stack: the scheduler waited for migrations, ran all
four commands on start (follow syncs as anonymous, similarity cleanup, and
a guest blacklist refresh that pulled the real 14-tag list from e621), and
kept looping. All six composes still validate.

deploy/.env.example

@@ -49,6 +49,12 @@ DB_ROOT_PASSWORD=j621root
 # GUNICORN_THREADS=4
 # GUNICORN_TIMEOUT=120
 
+# Scheduler intervals in seconds (the "scheduler" service runs the periodic
+# management commands; see deploy/README.md)
+# J621_SYNC_EVERY=1800
+# J621_CLEAN_EVERY=3600
+# J621_BLACKLIST_EVERY=86400
+
 # Rate limits (per IP anonymous, per account signed in)
 # THROTTLE_ANON=120/min
 # THROTTLE_USER=600/min