feat(scheduler): admin-tunable maintenance re-validation batch size

Expose the maintenance job's rolling re-validation batch (videos re-checked per
run) as an admin control on the Scheduler dashboard, stored in app_state
(migration 0018; NULL = the env/config default). The job reads the effective
value each run via a state helper, clamped to a sane range. Trilingual.

backend/app/state.py

@@ -3,10 +3,15 @@ import logging
 
 from sqlalchemy.orm import Session
 
+from app.config import settings
 from app.models import AppState
 
 log = logging.getLogger("subfeed.state")
 
+# Bounds for the admin-set maintenance re-validation batch (videos re-checked per run).
+MAINTENANCE_BATCH_MIN = 100
+MAINTENANCE_BATCH_MAX = 100_000
+
 
 def _row(db: Session) -> AppState:
     row = db.get(AppState, 1)
@@ -27,3 +32,20 @@ def set_sync_paused(db: Session, paused: bool) -> None:
     db.add(row)
     db.commit()
     log.info("Background sync %s", "paused" if paused else "resumed")
+
+
+def get_maintenance_batch(db: Session) -> int:
+    """Effective maintenance re-validation batch: the admin override if set, else the
+    env/config default."""
+    return _row(db).maintenance_revalidate_batch or settings.maintenance_revalidate_batch
+
+
+def set_maintenance_batch(db: Session, value: int) -> int:
+    """Clamp and persist the maintenance batch override. Returns the applied value."""
+    value = max(MAINTENANCE_BATCH_MIN, min(MAINTENANCE_BATCH_MAX, int(value)))
+    row = _row(db)
+    row.maintenance_revalidate_batch = value
+    db.add(row)
+    db.commit()
+    log.info("Maintenance re-validation batch set to %s", value)
+    return value