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/videos.py

@@ -9,6 +9,7 @@ from sqlalchemy import and_, func, or_, select, update
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session
 
+from app import progress
 from app.config import settings
 from app.models import Channel, Video
 from app.youtube.client import YouTubeClient, best_thumbnail
@@ -365,5 +366,6 @@ def run_shorts_classification(db: Session, limit: int | None = None) -> dict:
                 probed += 1
                 if result:
                     shorts += 1
+                progress.report(probed, len(candidates), "probe")
     db.commit()
     return {"probed": probed, "shorts": shorts}