fix(security): add cross-site gate to military-installations proxy

The /api/military-installations endpoint was the last keyless proxy without
the admitSameSite gate that protects terrain, CCTV, traffic, places, and
Celestrak. A cross-site <img>, foreign Origin, opaque origin, or
reverse-proxy-forwarded request now receives 403 before any Overpass query.
Same-origin app requests and bare tool requests continue to work.

Adds 4 integration tests (cross-site refused, same-origin admitted,
bare tool admitted, proxied refused) following the terrainHeightsBounds
test pattern.

server/providers/military-installations.js

@@ -1,4 +1,5 @@
 import { makeRateLimiter, clientKey } from './common/rate-limit.js';
+import { admitSameSite } from './common/same-site.js';
 import {
   MILITARY_INSTALLATION_ELEMENT_CAP,
   MILITARY_INSTALLATION_MAX_RESPONSE_BYTES,
@@ -97,6 +98,7 @@ function militaryInstallationsProxy() {
         res.end(JSON.stringify({ error: 'Method Not Allowed' }));
         return;
       }
+      if (admitSameSite(req, res)) return;
       const url = new URL(req.url, 'http://localhost');
       const requested = validMilitaryInstallationBox(url.searchParams);
       if (!requested) {