fc3d: cleanup_old_download_events daily task (terminal-only, configurable retention)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/tasks/maintenance.py

@@ -11,7 +11,7 @@ from sqlalchemy.orm import sessionmaker
 
 from ..celery_app import celery
 from ..config import get_config
-from ..models import ImageRecord, ImportTask
+from ..models import DownloadEvent, ImageRecord, ImportSettings, ImportTask
 from ..utils.phash import compute_phash
 
 log = logging.getLogger(__name__)
@@ -196,3 +196,29 @@ def verify_integrity() -> int:
             last_id = rows[-1].id
     log.info("verify_integrity verdicts: %s (total %d)", counts, total)
     return total
+
+
+@celery.task(name="backend.app.tasks.maintenance.cleanup_old_download_events")
+def cleanup_old_download_events() -> int:
+    """FC-3d: delete terminal DownloadEvent rows older than the configured
+    retention window. Never touches pending/running rows.
+
+    Why terminal-only: pending/running rows represent in-flight work whose
+    owning task may still be alive; deleting them would orphan the task.
+    Retention days comes from ImportSettings.download_event_retention_days
+    so the operator can tune without a code change.
+    """
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        settings = session.execute(
+            select(ImportSettings).where(ImportSettings.id == 1)
+        ).scalar_one()
+        retention_days = settings.download_event_retention_days
+        cutoff = datetime.now(UTC) - timedelta(days=retention_days)
+        result = session.execute(
+            delete(DownloadEvent)
+            .where(DownloadEvent.status.in_(["ok", "error", "skipped"]))
+            .where(DownloadEvent.started_at < cutoff)
+        )
+        session.commit()
+        return result.rowcount or 0