M1-T05: drop location/poo database config from Settings and tests

Remove the dead location_database_url / poo_database_url fields and the
location_sqlite_path / poo_sqlite_path computed properties from Settings;
drop them from the config-page payload and from .env.example. Update the
test hardcodes (test_config, test_public_ip, test_smtp) and reduce the
conftest test_database_urls fixture to the single app DB. The one-time
migration script keeps reading legacy URLs from env/CLI, independent of
Settings.

pytest 95 passed; ruff clean (pre-existing only).

tests/test_smtp.py

@@ -40,8 +40,6 @@ def _smtp_settings(**overrides) -> Settings:
         "app_env": "development",
         "app_hostname": "localhost:8000",
         "app_database_url": "sqlite:///./data/app.db",
-        "location_database_url": "sqlite:///./data/locationRecorder.db",
-        "poo_database_url": "sqlite:///./data/pooRecorder.db",
         "auth_bootstrap_username": "admin",
         "auth_bootstrap_password": "secret-password",
         "smtp_enabled": True,