Fix Base import in db/base.py for Alembic migrations

- Updated app/db/base.py to properly export Base class
- Fixed error when running Alembic migrations due to missing Base import
- Added proper imports for models

app/db/base.py

@@ -1 +1,6 @@
-# Import all models for Alembic to detect
+# Import all models and base for Alembic to detect
+from app.db.base_class import Base  # noqa
+from app.models.todo import Todo  # noqa
+
+# Export Base for other modules to use
+__all__ = ["Base"]