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]>

changelog.d/945-cctv-vegvesen-kartverket-heights.md

@@ -7,9 +7,11 @@
   time; a cold lookup takes about 6–13 s. Cameras over water use the water
   surface rather than the sea-floor depth the API returns there; for bridge
   cameras that is still below the deck. Heights are cached for good and misses
-  for 6 h, so a refresh only asks about new or moved cameras. After a 429, a
-  5xx or a network failure the lookup stops and backs off (Retry-After, else
-  10 min). The lane waits at most 12 s from its start; a slower first lookup
-  leaves the cameras it had not reached on the flat prior until the next
-  15-minute refresh, and keeps filling the cache meanwhile.
+  for 6 h, so a refresh only asks about new or moved cameras. After a
+  failing request (a 408, 429 or 5xx, a network error, a request over 10 s or
+  an unreadable body) the lookup stops and backs off (Retry-After, else
+  30 min, longer than the refresh). The lane waits at most 12 s from its
+  start, and only on a cold cache; a slower first lookup leaves the cameras it
+  had not reached on the flat prior until the next 15-minute refresh, and
+  keeps filling the cache meanwhile.
   `CCTV_VEGVESEN_HEIGHTS=0` turns it off. Follow-up to #929 (trygvehhh, #945).