feat(scheduler): every background run is a durable row

What a job last did lived in the API process's memory and was gone at
every restart; the audit log kept a run only when it was manual, failed
or changed something. Now each run is a job_runs row written at its
start and its end in its own session: who started it, its outcome as
data, what it spent of the YouTube quota (quota_events.job_run_id) and
the lines it logged (log_events.run_id, filterable in the log console).

- a run a stopped process left open is marked interrupted at startup
- "run now" is a one-shot job on the scheduler, which now always starts
  (interval jobs only with SCHEDULER_ENABLED); the manual actor is reset
  after the run, since pool threads are reused
- the dashboard's last run comes from job_runs, so it survives a deploy
- runs leave the audit log: migration 0109 moves the old job_run rows
  over (and back on downgrade); the audit page's run folding goes
- job_run_retention_days (default 30) prunes old runs in audit_gc

backend/app/audit.py

@@ -48,8 +48,6 @@ class AuditAction:
     MAINTENANCE_TUNE = "maintenance_tune"
     SYNC_PAUSE = "sync_pause"
     SYNC_RESUME = "sync_resume"
-    # Scheduler job outcome / run-summary (actor_id set = a manual "run now"; NULL = automatic)
-    JOB_RUN = "job_run"
     # Per-user download cookie/auth credential set or removed (the value is NEVER recorded)
     DOWNLOAD_CREDENTIAL_SET = "download_credential_set"
     DOWNLOAD_CREDENTIAL_DELETE = "download_credential_delete"