This commit implements automatic cleanup of feeds that haven't been successfully fetched in 3 days.
Changes:
- Add last_successful_fetch field to Feed model to track when feeds are successfully fetched (HTTP 200)
- Update parse_org_social() and validate_org_social_feed() to record successful fetches
- Add new periodic task cleanup_stale_feeds() that runs every 3 days and deletes inactive feeds
- Add comprehensive test suite (9 tests, all passing)
- Migration safely handles existing feeds by setting initial last_successful_fetch to protect them
Safety features:
- Feeds with last_successful_fetch = NULL are never deleted (protects legacy feeds)
- Only feeds older than 3 days are deleted
- Extensive logging of cleanup operations
- Initial migration sets timestamp for all existing feeds to prevent accidental deletion
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