core: migrate from custom JSON storage to standard Drizzle migrations to improve database reliability and performance

This replaces the previous manual JSON file system with standard Drizzle migrations, enabling:
- Proper database schema migrations with timestamp-based versioning
- Batched migration for faster migration of large datasets
- Better data integrity with proper table schemas instead of JSON blobs
- Easier database upgrades and rollback capabilities

Migration changes:
- Todo table now uses individual columns with composite PK instead of JSON blob
- Share table removes unused download share data
- Session diff table moved from database table to file storage
- All migrations now use proper Drizzle format with per-folder layout

Users will see a one-time migration on next run that migrates existing JSON data to the new SQLite database.