fix(logs): keep polling reads and a channel-less account out of the warnings

The reads the frontend polls (unread counts, /api/me, the download index,
sync status, subtitle progress) are logged at DEBUG: at INFO they were ~95%
of the stored log on prod. A scheduler job failure and a playlist sync
failure carry event codes, and a Google account without a YouTube channel
has an empty playlist list instead of a warning every six hours.

backend/app/scheduler.py

@@ -208,7 +208,7 @@ def _job(name: str, fn) -> None:
         _audit_run(db, actor_id, name, "ok", summary, result)
     except Exception as exc:
         db.rollback()
-        log.exception("job %s failed", name)
+        log.exception("job %s failed", name, extra=logs.event("scheduler.job.failed", job=name))
         err = str(exc) or exc.__class__.__name__
         _record(name, status="error", last_finished=_now_iso(), last_error=err, progress=None)
         if actor_id is not None: