Serves the IndexNow key file that proves we own this host.
IndexNow verifies a submission by fetching https://host/<key>.txt and
checking it contains the same key that was submitted. The file is derived
from the GAMEND_INDEX_NOW_KEY setting rather than written to disk, so
rotating the key is a config change and nothing can go stale.
A no-op when IndexNow is unconfigured, and it only ever answers the one exact path — every other request falls through untouched.