feat(scheduler): admin run-now/run-all triggers + live progress + completion notices

Add per-job "Run now" buttons and a "Start all now" button to the admin Scheduler
dashboard (admin-gated endpoints). Triggers run the job in a background thread
independent of its interval, refusing a concurrent run (409). While running, the
long jobs (maintenance, enrich, backfill, shorts) report live progress through a
decoupled contextvar sink, shown as a progress bar on the job row via the existing
4s poll. A manually-triggered run posts a completion notification to the triggering
admin's inbox (scheduled runs stay silent to avoid spam); the inbox renders the
"scheduler" type trilingually from type+data. While here, give the maintenance job
its missing dashboard label/description in all three languages.

backend/app/sync/maintenance.py

@@ -25,7 +25,7 @@ from datetime import datetime, timedelta, timezone
 from sqlalchemy import or_, select
 from sqlalchemy.orm import Session
 
-from app import quota
+from app import progress, quota
 from app.config import settings
 from app.models import Playlist, PlaylistItem, Video, VideoState
 from app.notifications import create_notification
@@ -147,6 +147,7 @@ def _recheck_flagged(db: Session, yt: YouTubeClient) -> dict:
     )
     if not flagged:
         return {"checked": 0, "recovered": 0, "deleted": 0, "notified": 0}
+    progress.report(0, len(flagged), "recheck")
     items = {it["id"]: it for it in yt.get_videos([v.id for v in flagged])}
     now = _now()
     recovered = 0
@@ -221,6 +222,7 @@ def _revalidate_rolling(db: Session, yt: YouTubeClient) -> dict:
                 flagged_new += 1
         db.commit()
         checked += len(page)
+        progress.report(checked, batch, "revalidate")
     return {"checked": checked, "flagged": flagged_new}