Attachment Sync Reliability Improvements

🔄 Improvements

• Improved attachment synchronization by automatically ignoring outdated thumbnail and auxiliary files that should no longer be transferred.
• Reduced unnecessary attachment re-sync operations, making synchronization more stable and efficient.

🛠 Fixes

• Fixed: stale attachment artifacts could reappear as separate sync items and trigger unnecessary transfers.
• Fixed: unchanged attachment folders could be repeatedly synchronized because outdated metadata was incorrectly detected as a difference.
• Fixed: scheduled file health checks could start duplicate runs within the same execution window.

And other improvements