fix: address review on Kartverket ground heights

- Send 20 points per request, not 50: above ~20 Kartverket answers the
  extra points from contour lines (hoydekurver) or not at all. At 20,
  nearly every camera resolves from dtm1, so the null-retry pass is gone.
  Eight requests in flight keep a cold lookup ~6 s.
- Cache misses: positions with no height are not asked again for 6 h.
- Transport failures: the first 429, 5xx or network error stops the
  lookup with no retries, and no lookup starts until the backoff ends
  (Retry-After when sent, else 10 min; bounded to 1 min-1 h).
- Budget the 12 s wait from the start of the loader, catalog fetch
  included.
- Record answers only for the points each request asked about.
- Send the CCTV User-Agent; link CC BY 4.0 in the credit.
- Docs: bridge cameras sit at the water surface, below the deck.
- Tests: batching, 8-in-flight cap, request count on failure, the shared
  deadline, Retry-After bounds, null TTL, backoff, the -20 m bound and
  each water marker on its own.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/dataCredits.js

@@ -246,7 +246,8 @@ export const DATA_CREDITS = [
       'Road cameras (Norway): contains data under the ' +
       '<a href="https://data.norge.no/nlod/en/2.0" target="_blank" rel="noopener">Norwegian licence for Open Government data (NLOD)</a> ' +
       'distributed by Statens vegvesen · camera ground heights © ' +
-      '<a href="https://kartverket.no/en/api-and-data/terrengdata" target="_blank" rel="noopener">Kartverket</a> (CC BY 4.0)',
+      '<a href="https://kartverket.no/en/api-and-data/terrengdata" target="_blank" rel="noopener">Kartverket</a> ' +
+      '(<a href="https://creativecommons.org/licenses/by/4.0/" target="_blank" rel="noopener">CC BY 4.0</a>)',
   },
   {
     key: 'gbfs',