Returns the VAPID public key for the frontend to pass into pushManager.subscribe({ applicationServerKey }).

Previously read STORYFLO_VAPID_PUBLIC_KEY via os.environ, which never matched the actual Fly secret name (VAPID_PUBLIC_KEY) — so this endpoint silently returned "" and the subscribe UI passed an empty applicationServerKey to pushManager.subscribe(). Now reads settings.vapid_public_key (the pydantic field that DOES match the Fly secret) so this legacy path returns the same value as /v1/push/vapid-public-key. Frontend caller at app/listen/share/page.tsx hits this URL.