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

server/providers/gbfs.js

@@ -1,4 +1,5 @@
 import { readResponseTextCapped } from './common/http.js';
+import { makeRateLimiter, clientKey } from './common/rate-limit.js';
 import {
   isAllowedGbfsHost,
   isAllowedGbfsPath,
@@ -13,6 +14,19 @@ const GBFS_PROXY_TIMEOUT_MS = 12000;
 
 export const GBFS_MAX_BODY_BYTES = 5 * 1024 * 1024; // 5 MB
 
+/**
+ * Requests/minute/IP admitted by the proxy.
+ *
+ * The bikeshare layer's own demand is small and known: `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` (60 s) with in-flight requests deduplicated. Even with every
+ * catalogued system in camera range at once that is about 15 requests a minute,
+ * so 120 is eight times a maximal real session while still capping a caller
+ * using this proxy as an open relay against a third-party GBFS host.
+ */
+export const GBFS_RATE_PER_MIN = 120;
+
 function gbfsRedirectHost(location, requestUrl) {
   if (!location) return '';
   try {
@@ -113,7 +127,12 @@ export async function fetchGbfsUpstream(
  *
  * @returns {import('vite').Plugin}
  */
-export function gbfsProxy() {
+export function gbfsProxy({ ratePerMinute = GBFS_RATE_PER_MIN } = {}) {
+  const allow = makeRateLimiter({
+    windowMs: 60_000,
+    max: ratePerMinute,
+    globalMax: ratePerMinute * 3,
+  });
   const installMiddleware = (server) => {
     server.middlewares.use('/api/gbfs', async (req, res) => {
       try {
@@ -126,6 +145,19 @@ export function gbfsProxy() {
           return;
         }
 
+        // Frequency, checked before the target is even parsed: the allowlist
+        // already says WHERE this proxy may go, nothing said HOW OFTEN. A
+        // refused request performs no outbound fetch and starts no timer.
+        if (!allow(clientKey(req))) {
+          res.writeHead(429, {
+            'Content-Type': 'application/json',
+            'Cache-Control': 'no-store',
+            'Retry-After': '10',
+          });
+          res.end(JSON.stringify({ error: 'Rate limit exceeded' }));
+          return;
+        }
+
         const url = new URL(req.url || '/', 'http://localhost');
         const encodedTarget = url.pathname.replace(/^\/+/, '');
         if (!encodedTarget) {