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>

artifacts/storage.py

@@ -7,9 +7,11 @@ SUM rather than a stat of every file.
 """
 
 import shutil
+import threading
 from datetime import timedelta
 
 from django.conf import settings
+from django.db import connection
 from django.db.models import F, Sum
 from django.utils import timezone
 
@@ -154,6 +156,45 @@ def purge_trashed(user):
     return len(trashed)
 
 
+TRASH_RETENTION = timedelta(days=3)
+
+
+def purge_expired_trash(now=None):
+    """Permanently remove trashed artifacts older than the retention window
+    (all users). Tolerant of already-gone rows so concurrent purges are safe.
+    Returns the number purged."""
+    now = now or timezone.now()
+    cutoff = now - TRASH_RETENTION
+    expired = list(Artifact.objects.filter(deleted_at__isnull=False, deleted_at__lt=cutoff))
+    purged = 0
+    for artifact in expired:
+        try:
+            delete_artifact_files(artifact)
+            artifact.delete()
+            purged += 1
+        except Exception:
+            # Another concurrent purge got there first, or a file's already
+            # gone — a purge error must never surface to anyone.
+            pass
+    return purged
+
+
+def spawn_trash_purge():
+    """Run purge_expired_trash off the request thread (cron-less: triggered by
+    deletes). Falls back to inline when async is disabled (tests)."""
+    if not getattr(settings, "AGENTFEED_ASYNC_PURGE", True):
+        purge_expired_trash()
+        return
+
+    def _run():
+        try:
+            purge_expired_trash()
+        finally:
+            connection.close()  # threads get their own connection; don't leak it
+
+    threading.Thread(target=_run, daemon=True).start()
+
+
 ASSET_ORPHAN_GRACE = timedelta(hours=24)