skip Nominatim lookups when the server has none configured

Add GET /api/geocode/status ({configured}), like /api/overpass/status. The outline client and the last-resort place search ask it once per page and send no lookup a server without a Nominatim endpoint would refuse, so a default install no longer logs a 503 on its first named landmark or grounds ask. An older server or a failed probe leaves the capability unknown and lookups proceed as before.

docs/CURRENT-STATE.md

@@ -5298,6 +5298,11 @@ Google first when configured, Photon/OpenStreetMap as fallback (including Google
 transport failures or declined requests), and the local `/api/geocode` route
 over Nominatim as a last resort when neither answers and an operator explicitly
 configures `NOMINATIM_URL`. Unset, empty and invalid values disable Nominatim.
+`GET /api/geocode/status` answers `{configured}`; the browser asks once per
+page, and when Nominatim is not configured neither the last-resort search nor
+the voice outline client asks `/api/geocode`, so a default install sends no
+lookup it would refuse. A missing or failed probe leaves the capability unknown
+and lookups proceed as before.
 Explicit public-service configuration retains an identifying `User-Agent` and `Referer`, at
 least 1.1 seconds between public search and outline starts, a shared durable
 per-install daily allowance and pause state that fail closed when unusable.