Refactor application imports and migration script: update model import paths in env.py and migrate.py, enhance app initialization in __init__.py, and modify PYTHONPATH in run.sh for improved directory structure.

Update Alembic configuration and migration scripts: change script location and version path in alembic.ini, add env.py for migration management, create initial migration script for audiobooks table, and implement migration execution in a new migrate.py script.