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/routes/scheduler.py

@@ -71,6 +71,29 @@ def run_all_now(user: User = Depends(admin_user)) -> dict:
     return {"started": trigger_all(actor_id=user.id)}
 
 
+@router.patch("/maintenance")
+def set_maintenance_settings(
+    payload: dict,
+    _: User = Depends(admin_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Admin-tune the maintenance job's rolling re-validation batch (videos re-checked per
+    run). Persisted to app_state; the job reads it each run."""
+    try:
+        value = int(payload.get("revalidate_batch"))
+    except (TypeError, ValueError):
+        raise HTTPException(status_code=400, detail="revalidate_batch must be a number")
+    if not (state.MAINTENANCE_BATCH_MIN <= value <= state.MAINTENANCE_BATCH_MAX):
+        raise HTTPException(
+            status_code=400,
+            detail=(
+                f"revalidate_batch must be between {state.MAINTENANCE_BATCH_MIN} and "
+                f"{state.MAINTENANCE_BATCH_MAX}"
+            ),
+        )
+    return {"revalidate_batch": state.set_maintenance_batch(db, value)}
+
+
 @router.get("")
 def get_scheduler(
     _: User = Depends(admin_user), db: Session = Depends(get_db)
@@ -138,4 +161,10 @@ def get_scheduler(
         "paused": state.is_sync_paused(db),
         "queue": queue,
         "quota": quota_info,
+        "maintenance": {
+            "revalidate_batch": state.get_maintenance_batch(db),
+            "revalidate_batch_default": settings.maintenance_revalidate_batch,
+            "min": state.MAINTENANCE_BATCH_MIN,
+            "max": state.MAINTENANCE_BATCH_MAX,
+        },
     }