refactor(config): fold maintenance_revalidate_batch from app_state into sysconfig (E3 S3, B-core-11)

The admin-tunable maintenance re-validation batch lived in its own app_state
column with a hand-rolled override-or-default reader in app.state — a fourth
config store duplicating the sysconfig SPECS pattern. Move it to a sysconfig key
(`maintenance` group, min=100 max=100_000): the reader (sync/maintenance.py) and
the Scheduler dashboard (routes/scheduler.py PATCH + GET) now go through
sysconfig.get_int / set_value / spec, and app.state loses get/set_maintenance_batch
+ the MIN/MAX constants + the settings import. Migration 0070 carries any existing
override into the system_config store before dropping the app_state column
(reversible; restores + copies back on downgrade). Reuses the existing
sysconfig.spec() for the dashboard's min/max, so the bounds have one source.

Full lane 365 pass (migration drift + reversibility exercised), ruff clean.

backend/app/sync/maintenance.py

@@ -25,11 +25,10 @@ from datetime import timedelta
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
-from app import progress, quota
+from app import progress, quota, sysconfig
 from app.config import settings
 from app.models import Playlist, PlaylistItem, Video
 from app.notifications import create_notification
-from app.state import get_maintenance_batch
 from app.sync.runner import get_service_user
 from app.sync.videos import apply_video_details, parse_dt
 from app.utils import now_utc as _now
@@ -182,7 +181,7 @@ def _recheck_flagged(db: Session, yt: YouTubeClient) -> dict:
 def _revalidate_rolling(db: Session, yt: YouTubeClient) -> dict:
     """Phase 2: re-check the least-recently-checked currently-available videos and flag any
     that have become unplayable (hidden immediately; deleted later if they don't recover)."""
-    batch = get_maintenance_batch(db)  # admin override, else config default
+    batch = sysconfig.get_int(db, "maintenance_revalidate_batch")  # admin override, else config default
     flagged_new = 0
     checked = 0
     now = _now()