fix(installations): send an identifying User-Agent to Overpass (#570)

Supersedes #420 by @GladiatorrX9, ported onto the current component layout with fixes from review.

Verified before merge: format, package boundaries, unit suite, build, and the browser tracking suite; a browser harness for the change where one applies; adversarial review applied.

Co-authored-by: Cyber Knight <[email protected]>

docs/CURRENT-STATE.md

@@ -2884,6 +2884,7 @@ silently demoting every later lookup for the session.
 - `/api/overpass` fans out across four public mirrors. `overpassPayloadIsData()` governs cache reads, writes, and stale fallback: only a 2xx that is neither rate-limited nor a body-level runtime error qualifies. Previously stored refusals are ignored on both fresh and stale reads, so upgrading does not require manually clearing the disk cache.
 - 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.
 
 ### Share-link v2 layer state (August 2026)