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/routes/scheduler.py

@@ -10,7 +10,15 @@ from app.config import settings
 from app.db import get_db
 from app.models import Channel, Subscription, User, Video
 from app.routes.admin import admin_user
-from app.scheduler import JOB_INTERVALS, MAX_INTERVAL, MIN_INTERVAL, apply_interval, scheduler_snapshot
+from app.scheduler import (
+    JOB_INTERVALS,
+    MAX_INTERVAL,
+    MIN_INTERVAL,
+    apply_interval,
+    scheduler_snapshot,
+    trigger_all,
+    trigger_job,
+)
 from app.sync.runner import estimate_deep_backfill
 
 router = APIRouter(prefix="/api/admin/scheduler", tags=["admin"])
@@ -40,6 +48,29 @@ def set_job_interval(
     return {"id": job_id, "interval_minutes": applied}
 
 
+@router.post("/jobs/{job_id}/run")
+def run_job_now(
+    job_id: str,
+    user: User = Depends(admin_user),
+) -> dict:
+    """Trigger a job to run immediately (in a background thread), independent of its
+    interval. 409 if it's already running, 404 for an unknown job. On completion the
+    triggering admin gets an inbox notification with the result."""
+    try:
+        result = trigger_job(job_id, actor_id=user.id)
+    except KeyError:
+        raise HTTPException(status_code=404, detail="Unknown job")
+    if result == "already_running":
+        raise HTTPException(status_code=409, detail="Job is already running")
+    return {"id": job_id, "started": True}
+
+
+@router.post("/run-all")
+def run_all_now(user: User = Depends(admin_user)) -> dict:
+    """Trigger every job that isn't already running. Returns the ids actually started."""
+    return {"started": trigger_all(actor_id=user.id)}
+
+
 @router.get("")
 def get_scheduler(
     _: User = Depends(admin_user), db: Session = Depends(get_db)