fix(cctv): harden the Kartverket height lookup for Vegvesen cameras

- Default backoff is 30 min, longer than the 15-minute catalog refresh,
  so a failing Kartverket costs one attempt per backoff, not per refresh.
- A point a 200 answer leaves out is cached as no height (6 h TTL)
  instead of being asked again, and waited on, every refresh.
- Only 408, 429 and 5xx stop the lookup; any other 4xx marks that
  request's points as no height and the lookup carries on.
- Each request has its own 10 s timeout inside the shared 30 s deadline;
  a lookup cut short only by the deadline resumes without a backoff.
- The lane waits only on a cold cache (>10% of cameras missing); a few
  new cameras or expired misses are looked up in the background.
- applyVegvesenGroundHeights skips cameras without finite coordinates,
  so it really never throws.
- clearKartverketHeightCache bumps a generation, so a lookup started
  before a clear cannot write to the cache, backoff or pending slot.
- Credit and DATA_SOURCES note the modification (water set to 0 m,
  rounded to 0.1 m) as CC BY 4.0 asks; docs match the stop rules.
- Tests for the deadline being shared, per-request timeouts, 503,
  oversized and non-JSON bodies, 4xx, left-out points, Retry-After end
  to end, overlapping refreshes, the warm-cache wait, bad coordinates,
  the cache-clear generation and the loader's wait budget.

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

src/data/dataCredits.js

@@ -295,7 +295,8 @@ export const DATA_CREDITS = [
       '<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> ' +
-      '(<a href="https://creativecommons.org/licenses/by/4.0/" target="_blank" rel="noopener">CC BY 4.0</a>)',
+      '(<a href="https://creativecommons.org/licenses/by/4.0/" target="_blank" rel="noopener">CC BY 4.0</a>); ' +
+      'modified: heights over water set to 0 m and rounded to 0.1 m',
   },
   {
     key: 'nyc-cctv',