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/models.py

@@ -339,6 +339,9 @@ class AppState(Base):
     sync_paused: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default="false"
     )
+    # Admin override for the maintenance job's rolling re-validation batch (videos re-checked
+    # per run). NULL = use the config/env default (settings.maintenance_revalidate_batch).
+    maintenance_revalidate_batch: Mapped[int | None] = mapped_column(Integer)
 
 
 class SchedulerSetting(Base):

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,
+        },
     }

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

backend/app/sync/maintenance.py

@@ -29,6 +29,7 @@ from app import progress, quota
 from app.config import settings
 from app.models import Playlist, PlaylistItem, Video, VideoState
 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.youtube.client import YouTubeClient
@@ -189,7 +190,7 @@ 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 = settings.maintenance_revalidate_batch
+    batch = get_maintenance_batch(db)  # admin override, else config default
     flagged_new = 0
     checked = 0
     now = _now()