Plumb a per-resource owner (Zitadel sub) through the sync contract without
enforcing isolation client-side — the server stays the authority.
- Dtos: add optional ownerId to RemoteList/RemoteTask/MirrorTask
- JwtClaims: decode the sub claim from the access token (never throws)
- OnlineSyncService: stamp ownerId on pushed lists + mirror; defensively skip
pulled tasks owned by a different user (unowned tasks still sync, so
single-user behavior is unchanged)
- docs: contract documents ownerId + multi-user readiness
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
