- Make client_id query parameter optional in /api/reports/last-sync
- When client_id is not provided, return sync information for all clients associated with the authenticated user's API token
- Add requires_full_sync flag to each client's response when no records exist across ALL report tables
- Update REPORTS_SYNC_PROTOCOL_DOCUMENTATION.md with checking last sync status section
- Add GET LAST SYNC INFORMATION section to REPORTS_SYNC_API_SPECIFICATION.txt

This fix allows clients to query their sync status without needing to know their client_id, making the API more user-friendly and enabling automatic recovery detection.