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/scheduler-entrypoint.sh

@@ -0,0 +1,62 @@
+#!/bin/sh
+# Periodic maintenance for a J621 deployment (runs in the "scheduler"
+# service; no host cron involved).
+#
+#   sync_followed_tags + sync_followed_pools   every SYNC_EVERY seconds   (30 min)
+#   cleanup_similarity                         every CLEAN_EVERY seconds  (1 h)
+#   refresh_guest_blacklist                    every BLACKLIST_EVERY      (24 h)
+#
+# Waits for the database and migrations to be ready, runs everything once,
+# then keeps checking. A failing command is logged and retried next interval,
+# so a temporary e621 outage is not fatal.
+set -u
+
+SYNC_EVERY="${SYNC_EVERY:-1800}"
+CLEAN_EVERY="${CLEAN_EVERY:-3600}"
+BLACKLIST_EVERY="${BLACKLIST_EVERY:-86400}"
+
+log() { echo "[scheduler] $(date -Iseconds) $*"; }
+
+run() {
+    log "running: $*"
+    if "$@"; then
+        log "done: $*"
+    else
+        log "FAILED (retrying at the next interval): $*"
+    fi
+}
+
+command -v python >/dev/null || { log "python not found"; exit 1; }
+
+log "waiting for the database and migrations..."
+until python manage.py migrate --check >/dev/null 2>&1; do
+    sleep 10
+done
+log "database ready; intervals: sync=${SYNC_EVERY}s cleanup=${CLEAN_EVERY}s blacklist=${BLACKLIST_EVERY}s"
+
+now=$(date +%s)
+last_sync=$((now - SYNC_EVERY))
+last_clean=$((now - CLEAN_EVERY))
+last_blacklist=$((now - BLACKLIST_EVERY))
+
+while true; do
+    now=$(date +%s)
+
+    if [ $((now - last_sync)) -ge "$SYNC_EVERY" ]; then
+        last_sync=$now
+        run python manage.py sync_followed_tags
+        run python manage.py sync_followed_pools
+    fi
+
+    if [ $((now - last_clean)) -ge "$CLEAN_EVERY" ]; then
+        last_clean=$now
+        run python manage.py cleanup_similarity
+    fi
+
+    if [ $((now - last_blacklist)) -ge "$BLACKLIST_EVERY" ]; then
+        last_blacklist=$now
+        run python manage.py refresh_guest_blacklist
+    fi
+
+    sleep 30
+done