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]>

.env.example

@@ -195,6 +195,7 @@ AISSTREAM_API_KEY=
 # CCTV_VEGVESEN_ENABLED=1                              # set 0 to disable the Statens vegvesen (Norway) road cameras
 # CCTV_VEGVESEN_MAX_SOURCES=900                        # cap cameras loaded from Statens vegvesen (default loads all ~850 live; max 1000)
 # CCTV_VEGVESEN_VIDEO=1                                # set 0 to show stills instead of live HLS for Statens vegvesen video cameras
+# CCTV_VEGVESEN_HEIGHTS=1                              # set 0 to skip Kartverket ground heights for Statens vegvesen cameras (flat prior instead)
 # CCTV_VEGVESEN_URL=                                   # override the Statens vegvesen catalog URL
 # CCTV_MAX_SOURCES=4000                                # catalog-wide ceiling, shared round-robin across packs
 # CCTV_PREFER_AUSTIN=1                                 # prioritize Austin cameras in the registry