Allow users to sync missing snapshots from the source repository to a
mirror. A new sync button per mirror opens a dialog showing which
snapshots are missing, with checkboxes to select which ones to copy.
- Add GET /:shortId/mirrors/:mirrorShortId/status endpoint to compare
snapshots between source and mirror repositories
- Add POST /:shortId/mirrors/:mirrorShortId/sync endpoint accepting
selected snapshotIds in the request body
- Extend restic copy command to accept multiple snapshotIds
- Add sync preview dialog with snapshot selection to the frontend
