internal/localapi: real /backup/due (cadence; due when no successful backup or
newest older than backup.backup_cadence_seconds; false in-window after success;
failed doesn't count) + /backup/status phases (idle|running|done|failed + job
id) + POST /backup single-flight with job id. Drives the controller quiesce loop.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>