Storage: migrate from Markdown back to SQLite

[ComputelessComputer]

The current filesystem-based storage (markdown + JSON) makes local-cloud file sync impossible to implement correctly. Moving back to SQLite restores a proper database layer while keeping the product local-first, and enables cross-device sync via SQLite replication.

Related

• Local-cloud file sync via Char Cloud #3421 — Local-cloud file sync via Char Cloud (enabled by this migration)
• Save notes to Markdown rather than SQLite #1355 — Original move from SQLite to Markdown (being reversed)