investigate scan and rescan completion time #111

Problem

For the home-syncthing folder with 136,624 files, scanning remained visible for
several minutes before completing without errors.

Syncthing was unpaused at 21:16:15 and logged Completed initial scan at
21:19:43. Its REST status later reported idle at 21:23:07.

Investigate

• measure normal rescan time separately from an initial scan after resume
• distinguish Syncthing scan time from delayed Syncshell completion feedback
• verify events and snapshots clear the rescan tracker promptly
• add elapsed feedback only if multi-minute scans are expected