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/J621-Backend

@@ -28,8 +28,9 @@ RUN pip install --no-cache-dir -r requirements.txt
 
 COPY backend/ ./
 COPY deploy/backend-entrypoint.sh /usr/local/bin/j621-entrypoint
+COPY deploy/scheduler-entrypoint.sh /usr/local/bin/j621-scheduler
 
-RUN chmod +x /usr/local/bin/j621-entrypoint \
+RUN chmod +x /usr/local/bin/j621-entrypoint /usr/local/bin/j621-scheduler \
     && mkdir -p /app/media /app/logs \
     && python manage.py collectstatic --noinput