feat(rust): SSH tunnel via russh #1894

Parent

#1850

Goal

Open local forward through russh for SQL driver connections.

Done when

• Unit/integration with mock or documented manual tunnel test

How to work

• Branch: issue/<n>-slug from epic/tauri-migration (create epic branch if missing).
• One PR → Closes #<n>.
• Target PR base: epic/tauri-migration until the SQLite slice is green.
• Do not edit Flutter app code unless this issue says so.