Periodic backups remain active after restoring a backup

Restoring a backup tells the user that automatic syncing and backups are disabled until the profile is closed. The shutdown-backup path honors `restoring_backup`, but the periodic-backup timer does not.

Reproduction (use a disposable profile and backup):
1. Restore a backup and keep the recovered profile open.
2. Make a collection change, with periodic backup timing configured so that a backup becomes due.
3. Allow the periodic-backup timer to fire (it checks every five minutes).

Expected: automatic backups remain suppressed during the recovery session. A user-requested backup remains available.
Actual: the timer requests an automatic backup, which can create a new snapshot when the backend's time/change conditions are satisfied.

Source diagnosis: `_start_restore_backup()` sets `self.restoring_backup = True`, and it remains set until profile unload. `_unloadCollection()` and automatic sync already check it. `on_periodic_backup_timer()` calls `_create_backup_with_progress(user_initiated=False)` unconditionally.

A deterministic reproduction calls the timer method with `restoring_backup=True` and records the backup request; this avoids waiting or touching a real collection. The fix belongs in the timer so manual backups remain available.