fix(traffic): say which upstream declined a road load

Street Traffic draws on OpenStreetMap for geometry and TomTom for flow,
and the row said "Road data temporarily unavailable" whichever one
failed. The reporter of #661 went and verified a TomTom key while every
public Overpass mirror was refusing their network with HTTP 406 — a
status fetchRoads already had and dropped on the floor.

Classify the refusal where the layer talks to Overpass: 429 is
"Overpass rate-limited", 504 is "Overpass timed out", anything else is
"Overpass refused the road query (HTTP <status>)". The first two are the
words alpr's Overpass source already uses; the status is what says which
upstream declined, which a single-source layer never has to.

The error carries its status as a RoadRequestError, following
LiveSourceError in src/sources/live/contract.js, so a caller can branch
on the code without parsing English. The catch promotes a classified
refusal and keeps the general line for everything else — the branch is
on the error type, not on its text, because a dropped socket throws
whatever the platform felt like saying and "Failed to fetch" on a panel
row helps nobody.

Retries, backoff, caching and the mirror rotation are untouched.

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+- Report which upstream declined a Street Traffic road load. The layer row now
+  reads `Overpass rate-limited`, `Overpass timed out`, or
+  `Overpass refused the road query (HTTP 406)` instead of a general
+  "Road data temporarily unavailable", so a reader is not sent to check a
+  TomTom key when the public OpenStreetMap mirrors are the side that failed.
+  Failures the layer cannot classify keep the general line.
+
 - Enable responsive trackpad pinch zoom on the globe. Browser pixel-mode
   `Ctrl+wheel` pinch gestures now reach Cesium with bounded amplification,
   while ordinary wheel, line-mode and touch-pinch inputs retain their existing