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/logs.py

@@ -10,8 +10,8 @@ the logs were both unjoinable and leaky. `configure_logging()` replaces all four
 Event shape — the OpenTelemetry log data model, without the SDK; one JSON object per line:
     ts · level · sev · source · logger · event · msg · attrs
 plus whatever correlation the context carries (`request_id`, `user_id`, `debug_session_id`,
-`job_id`) and the app version. That shape is what the admin log console filters on and what a Gitea
-issue attachment will carry, so it is deliberately flat and stable.
+`job_id`, `run_id`) and the app version. That shape is what the admin log console filters on and
+what a Gitea issue attachment will carry, so it is deliberately flat and stable.
 
 Call sites stay ordinary stdlib logging:
 
@@ -81,15 +81,23 @@ _debug_session: contextvars.ContextVar[str | None] = contextvars.ContextVar(
     "log_debug_session", default=None
 )
 _job_id: contextvars.ContextVar[int | None] = contextvars.ContextVar("log_job_id", default=None)
+# The background run (app/jobruns.py) this context is doing; quota's spend is tagged with it too.
+_run_id: contextvars.ContextVar[int | None] = contextvars.ContextVar("log_run_id", default=None)
 
 _CONTEXT_VARS = {
     "request_id": _request_id,
     "user_id": _user_id,
     "debug_session_id": _debug_session,
     "job_id": _job_id,
+    "run_id": _run_id,
 }
 
 
+def current_run_id() -> int | None:
+    """The background run this context is doing, if any — `bind(run_id=…)` sets it."""
+    return _run_id.get()
+
+
 class _RequestScope:
     """What the auth dependency learns about a request, for every line logged after it.