Backup SQLite DB before running migrations

[reddpy]

Problem

src/main/db.ts runs schema migrations directly on the user's only DB. Each migration is wrapped in a transaction, so a failed ALTER TABLE rolls back — but:

• A power loss mid-migration can leave a corrupted DB
• A migration bug that passes the transaction but produces wrong data has no rollback artifact
• Users have no recovery path other than losing all notes

Fix

Before running any pending migration:

1. Read current schema_version
2. If pending migrations exist, copy lychee.sqlite3 to lychee.sqlite3.bak-v{n} in the same directory
3. Keep the last N (e.g. 3) backup files, prune older
4. On migration failure, log the backup path so user/support can restore

Optional: add a "Restore from backup" entry in Settings that lists available backups.

Acceptance

• After upgrading from v8 to v9, a lychee.sqlite3.bak-v8 file exists in userData
• Forcing a migration to throw leaves the original DB usable (transaction rollback) AND leaves the backup file