"""SQLite offline outbox + monotonic sync cursor.

Undelivered frames are appended per ``chat_id`` with a monotonic cursor so a
reconnecting app can ``sync {cursor}`` the delta without re-reading full
history. Retention prunes old entries (``outbox_retention_hours``).

Storage: ``get_hermes_home()/"android"/outbox.db``.

Milestone M3 (built), extended in M5 (push integration).
"""