ImportError: ImportError: cannot import name 'BaseSettings' from 'pydantic'

Root cause

Pydantic v2 no longer exports BaseSettings from pydantic; BaseSettings was moved into the separate pydantic-settings package, while app/core/config.py still uses the Pydantic v1 import path.

Minimal fix

Add pydantic-settings to the project dependencies, then update app/core/config.py to import BaseSettings from pydantic_settings while keeping other Pydantic imports such as Field from pydantic, e.g. `from pydantic_settings import BaseSettings` and `from pydantic import Field`. Re-run `pytest tests/test_config.py -q` to verify.