v0.5.1: automatic Trash purge (cron-less background thread)

Trashed artifacts past a ~3-day retention window are now purged automatically.

- storage.purge_expired_trash(): removes trash older than TRASH_RETENTION
  (all users), tolerant of already-gone rows (concurrent-safe).
- spawn_trash_purge(): runs it on a daemon thread (manages/closes its own DB
  connection), triggered by each delete so it's cron-less and off the request
  thread. Inline fallback when AGENTFEED_ASYNC_PURGE is off (tests).
- Wired into both delete paths (API + session). Manual Empty Trash unchanged.
- Tests: purge removes expired-only; delete triggers the purge. 152 passing;
  live delete verified with the thread enabled (204, server healthy).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/api/test_artifacts.py

@@ -17,6 +17,7 @@ from artifacts.models import ApiKey, Artifact
 User = get_user_model()
 
 
+@override_settings(AGENTFEED_ASYNC_PURGE=False)  # DELETE triggers purge; run it inline in tests
 class ArtifactAPITestCase(APITestCase):
     """Exercises the ingest/list/detail API against a real (temp) filesystem
     and a real (test) database, the way an agent posting over HTTP would."""