Migrate to `UUID` from PostgreSQL's serial auto incrementing ID

[myndaaa]

Currently, the SQLAlchemy models use Integer as the primary key (id) for most tables, including sensitive user-facing ones like User, Playlist, Song, etc. This exposes internal sequence and can lead to security/privacy concerns when surfaced in public APIs (e.g., GET /users/17).

Replacing id: Integer with UUID:

• Changing primary keys to UUID(as_uuid=True) using default=uuid.uuid4.
• Updating Alembic migrations, Pydantic schemas, and CRUD logic accordingly.
• Benefits: no ID guessing.