Add migrations system for database schema management ...

- Create SHServ\Migrations\Migration base class with up()/down()
- Create MigrationsManager: tracks applied migrations in  table,
  supports migrate, rollback, status, and create commands
- Integrate with console.php: migrate, migrate:rollback, migrate:status,
  migrate:create
- Add first migration: create_sessions_table (needed for Session Entity)
- Use correct ThinBuilder field format for defaults and nullability