refactor: separate storage, migration, sync, sync merger

[sftblw]

Currently There's only a single script which handles all migration, syncing against local and sync storage, blahblah... It makes me headaching.

These should be separated into separate concerns in form of classes, before going further.

• JSON <-> object mapping
• JSON migrator from old to new version
• "Repository pattern"
• syncing between local and sync storage