fix: keep the Mentions Inbox across Gateway restarts (#141404)

fix: keep the Mentions Inbox across Gateway restarts (#141404)

Persist bounded mention sources and consumption receipts through existing
machine-state records, with atomic revision checks and publication after
commit. Preserve original IDs and expiry, current authorization, incognito
exclusion, and startup-owned cleanup without replaying old pushes.

No SQLite tables, columns, indexes, triggers, schema versions, configuration
options, or dependencies change. This implements the maintainer-approved
schema-free persistence scope following #141081.

Proof: red/green durability, expiry, rollback, cross-instance and unchanged
schema regressions; Gateway sibling tests; changed checks; UI build; real
Gateway SIGKILL/restart with retained IDs, expiry and dismissal. Independent
review's merge-order concern was rejected against the existing null-wins
branches and regression proof for both orders and restart.