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/config.py

@@ -268,6 +268,9 @@ class Settings(BaseSettings):
     # YouTube quota history (quota_events): days kept (0 = forever). A year, so the admin usage view's
     # longest window (365 days) stays whole; the same daily cleanup job prunes it.
     quota_retention_days: int = 365
+    # Background-run history (job_runs): days kept (0 = forever). Every run is a row (~1–1.5k a day
+    # at the default intervals), so a month; the same daily cleanup job prunes it.
+    job_run_retention_days: int = 30
     # On-disk layout: "plex" (Channel/Season YYYY/… [id].ext + .nfo/poster) or "flat".
     download_layout: DownloadLayout = "plex"
     # Default SponsorBlock (skip sponsor segments) for new custom profiles.