feat(store): add an incremental sync engine driven by server sync ids

Add `sync/sync-engine.ts`, which keeps the last applied sync id in a new
`sync_meta` SQLite table (migration 0039), bootstraps from `/sync/state`
plus the existing snapshot endpoints, and then pages through `/sync/delta`
on launch, foreground, reconnect, a 60 s interval and after the transaction
queue receives an acknowledgement. Subscription, collection and timeline
actions are applied to the stores and SQLite, pending local marks keep
precedence through the transaction queue overlays, and unread counts are
refreshed once per pull. A 404 from the sync endpoints keeps today's full
refetch behaviour for the session.

Both apps expose the endpoints through `followClient.request` until the
client SDK release with the `sync` module lands.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>