Replace manual if-sqlite/if-postgres branching with Alembic:
- Add alembic dependency
- Create programmatic alembic env (no CLI/alembic.ini needed)
- Support async engines via run_sync passthrough
- render_as_batch=True for SQLite ALTER TABLE compatibility
- Auto-stamp baseline on first run (existing DB at version 25)
- Run alembic upgrade head after legacy migrations
- Include sample migration showing schema + data migration patterns
- Add alembic dir to package-data for distribution
