clockula-backup-v1.json, schema documented in docs/BACKUP.md per
PLAN.md §7: alarms, timers and world clocks as configuration only —
ring state and a timer's running anchors never leave the device, since
neither means anything on another one. Unknown fields are ignored at
any level; a malformed known field rejects the whole file rather than
importing it half-way.
Import is whole-state replacement, not a merge, and the backup screen
says so before the user confirms. Before replacing: a ringing alarm is
dismissed and every running timer is deleted, so the import can never
strand a live ring session; after it, AlarmEngine and TimerEngine are
told to re-resolve so the imported state is live immediately.
BackupDao adds delete-all/insert-many queries under one transaction —
no schema or version change, queries only.
