feat: improve importer stability and manual queue prioritization

[drondeseries]

This PR introduces several stability and usability enhancements to the importer and queue system:

• Manual Queue Prioritization: Added a "Start Now" capability that allows manually triggered jobs to bypass worker limits and process immediately.
• Robust FUSE Cleanup: Enforced a more reliable cleanup process for FUSE mounts.
• ARR Webhook Protection: Added logic to ignore deletion webhooks from Sonarr/Radarr to prevent accidental data/mount loss during automated cleanup cycles.
• VFS Stability: Reverted propagation logic to a known stable version (May 15) to resolve recent intermittent issues.