Content Syndication 1.0.2 - Security Review Ready

Final fixes for security application readiness:
- Rate limit: use DelayedRequeueException to prevent cron spin-out
- Graph API: documentation updated to v21.0 (matches actual implementation)
- Tests: functional test field access fixed (nid instead of getNodeId)
- Libraries: removed unused/missing asset declarations

All code audit findings resolved. Ready for Drupal security team review.