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/runstatus.py

@@ -0,0 +1,35 @@
+"""The words a background run's row is written with — the `job_runs` table's status and trigger.
+
+One source for the literals: the scheduler writes them, the startup sweep rewrites `running`, the
+model's CHECK constraints are built from these sets, and the dashboard reads them back. A run's
+status is never typed as a string anywhere else (the download pipeline's `cancelled`/`canceled`
+drift, `downloads/status.py`, is why).
+"""
+
+from enum import StrEnum
+
+
+class RunStatus(StrEnum):
+    RUNNING = "running"
+    OK = "ok"
+    ERROR = "error"
+    # Sync was paused: the run was due and deliberately did nothing.
+    SKIPPED = "skipped"
+    # The process stopped while the run was going (a deploy, a crash): set at the next startup, so a
+    # row never claims to be running in a process that no longer exists.
+    INTERRUPTED = "interrupted"
+
+
+class RunTrigger(StrEnum):
+    # The run's interval came round.
+    SCHEDULED = "scheduled"
+    # An admin pressed "run now" (or "run all").
+    MANUAL = "manual"
+    # A user's own action started work on one target (a channel's full history), outside the
+    # recurring jobs.
+    AD_HOC = "ad_hoc"
+
+
+def sql_in(values: type[StrEnum]) -> str:
+    """`'a', 'b'` — the IN-list a CHECK constraint names these values with."""
+    return ", ".join(f"'{v.value}'" for v in values)