GET /api/data/export serializes all integrations (with decrypted secrets, for
cross-instance portability), bookmark categories, bookmarks, and tunnels;
POST /api/data/import restores them additively in a transaction with old->new
id remapping. Wires the Settings "Data & Backup" section to download/upload the
backup file. Verified end-to-end including cross-instance portability under a
different ARCHNEST_SECRET_KEY, plus browser verification of the Settings UI.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BbJV5nm8KPVH1oNJYKpnoF
5b17bba80e