fix(gbfs): rate-limit /api/gbfs

The proxy already restricted WHERE it would go — https, an allowlisted host, a
station_information/station_status path, no redirects, a 12-second deadline and
a 5 MB body cap — but nothing restricted HOW OFTEN, so the dev server would
relay an unbounded stream of requests at a third-party bikeshare operator on
any caller's behalf, one outbound fetch and one timeout timer each.

Admit 120 requests/minute per client address, global backstop 3x, refused with
429 and `Retry-After` before the target is even parsed. The ceiling comes from
the layer's own demand: `src/layers/bikeshare/registry.js` catalogues 15
systems, station information is cached per city after its first fetch, and
station status refreshes once per city every STATUS_POLL_MS (60s) with
in-flight requests deduplicated — about 15 requests a minute with everything
in range, so 120 is eight times a maximal session.

The method, host, path, redirect, deadline and size guards are unchanged.

Refs #264

scripts/format-scope.json

@@ -839,5 +839,6 @@
   "src/ui/imageryBoxTool.test.mjs",
   "src/ui/recentImagery.test.mjs",
   "src/layers/recentImagery/testDoubles.mjs",
-  "scripts/qa-recent-imagery.mjs"
+  "scripts/qa-recent-imagery.mjs",
+  "src/data/gbfsProxyRateLimit.test.mjs"
 ]