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).

app/services/config_page.py

@@ -260,8 +260,6 @@ def _settings_payload(settings: Settings) -> dict[str, Any]:
         "app_debug": settings.app_debug,
         "app_hostname": settings.app_hostname,
         "app_database_url": settings.app_database_url,
-        "location_database_url": settings.location_database_url,
-        "poo_database_url": settings.poo_database_url,
         "ticktick_client_id": settings.ticktick_client_id,
         "ticktick_client_secret": settings.ticktick_client_secret,
         "ticktick_token": settings.ticktick_token,