feat: Kartverket ground heights for Statens vegvesen cameras

The Vegvesen camera feed is 2D, so every camera shared one flat 150 m
ground prior and mountain-pass cameras could sit hundreds of metres below
the terrain where neither the point-height prior nor the ground snap
resolves. Each camera now gets its terrain height from Kartverket's
keyless point-height API (CC BY 4.0): ~840 of ~845 cameras.

- 50 points per request, six in flight; points a full batch answers null
  for are retried once in batches of 10 (the API drops points from large
  batches).
- Sea-floor depths (dybdekurver / Havflate) under bridges and quays
  count as the water surface, 0 m.
- Heights are cached by position, so the 15-minute refresh only asks
  about new or moved cameras. A catalog load waits at most 12 s; a slower
  lookup keeps filling the cache for the next refresh.
- CCTV_VEGVESEN_HEIGHTS=0 turns it off; Kartverket is credited beside
  Statens vegvesen in the attribution popover.

Follow-up to #929.

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

src/data/dataCredits.js

@@ -245,7 +245,8 @@ export const DATA_CREDITS = [
     html:
       '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',
+      '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)',
   },
   {
     key: 'gbfs',