sync_tombstones logs every owner-scoped delete on the synced tables so an
offline device can apply it later; filtered Realtime channels never deliver
DELETE, so the tombstone INSERT is also the live deletion signal.
history.revision now follows content changes made without a revision (the
desktop mirror), meeting_memos gains updated_at, and cursor indexes back
keyset pulls. Templates get client-keyed upsert/delete wrappers over the
revision-checked mobile RPCs.
export_account_portability serialised whole rows while the v1 archive is an
exact key set; columns added later made every account with meetings fail
export. Each dataset is projected back onto its v1 keys.
