fix(traffic): name the proxy's own 502/503 accurately (#665)

502 (every mirror unreachable) and 503 (the local limiter, before any mirror
is asked) come from our proxy, so they no longer read as a mirror refusing
the query. Credit the fix in CHANGELOG and note the mapping in CURRENT-STATE.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

docs/CURRENT-STATE.md

@@ -3483,7 +3483,7 @@ silently demoting every later lookup for the session.
 - HTTP refusals such as 406 now rotate alongside the existing network, rate-limit, and runtime-error cases. A refusal from one mirror no longer prevents reaching healthy alternatives or persists under the seven-day road/month-long boundary cache TTLs. Concurrent identical queries share one mirror sequence; if it fails, both the initiating and joined callers can use the same last-good data.
 - A refusal every mirror agrees on is still reported with the first mirror's status and body, so a genuinely malformed query says what upstream said — but only after every mirror has had the chance to answer it. `fetchOverpassPayload` takes injectable endpoints and fetch so the rotation is tested without a live mirror (`src/overpassProxy.test.mjs`).
 - Every mirror is asked with a User-Agent that names the application, its version and the project address (`OVERPASS_USER_AGENT` in `server/providers/overpass/constants.js`), which is what the OSM API usage policy asks for. Mirrors may refuse a client they cannot identify; such a refusal is never treated as data and costs the fan-out that mirror, so the header is what keeps the list at full strength.
-- When the fan-out has nothing left to offer, the traffic row says which upstream declined and how: `Overpass rate-limited`, `Overpass timed out`, or `Overpass refused the road query (HTTP 406)`. Street Traffic draws on two upstreams — OpenStreetMap for geometry, TomTom for flow — so a row that only said "road data unavailable" sent readers to check a TomTom key that was never the problem (#661). `roadRequestError` in `src/layers/traffic/source.js` owns the mapping and tags the error with its status. A refusal whose code is unreadable — the layer takes its source by injection, so an adapter may not supply one — reports `Overpass temporarily unavailable` rather than printing `HTTP undefined`, and anything the layer did not classify at all (a dropped connection, a malformed snapshot) still reads `Road data temporarily unavailable`, because a platform exception is not a sentence to put on a panel.
+- When the fan-out has nothing left to offer, the traffic row says which upstream declined and how: `Overpass rate-limited`, `Overpass timed out`, or `Overpass refused the road query (HTTP 406)`; the proxy's own 502 (every mirror unreachable) and 503 (local limiter busy) read `Overpass mirrors unreachable` and `Overpass temporarily unavailable`. Street Traffic draws on two upstreams — OpenStreetMap for geometry, TomTom for flow — so a row that only said "road data unavailable" sent readers to check a TomTom key that was never the problem (#661). `roadRequestError` in `src/layers/traffic/source.js` owns the mapping and tags the error with its status. A refusal whose code is unreadable — the layer takes its source by injection, so an adapter may not supply one — reports `Overpass temporarily unavailable` rather than printing `HTTP undefined`, and anything the layer did not classify at all (a dropped connection, a malformed snapshot) still reads `Road data temporarily unavailable`, because a platform exception is not a sentence to put on a panel.
 
 ### Share-link v2 layer state (August 2026)