fix(server): admit reverse-proxied same-origin terrain requests

Review follow-up on #975. The shared same-site gate refuses any forwarding header, which is right for routes that spend a key, but terrain is keyless and needed for rendering: behind the reverse proxy the README documents, every /api/terrain/heights chunk got 403 and fell back to the geoid. Terrain now uses admitSameSiteKeyless, which keeps the Sec-Fetch-Site and exact-Origin checks (comparing Origin against a single X-Forwarded-Proto scheme) but does not refuse forwarding headers. The shared admitSameSite gate is unchanged for every other route. The changelog entry moves to changelog.d/975-terrain-cross-site-gate.md.

Co-authored-by: daikaginza <[email protected]>

changelog.d/975-terrain-cross-site-gate.md

@@ -0,0 +1,8 @@
+- Cross-site pages can no longer fill the terrain-height cache or spend the
+  visitor's own `/api/terrain/heights` rate-limit quota through the visitor's
+  browser. The route refuses a cross-site `Sec-Fetch-Site` or a foreign or
+  opaque `Origin` before rate limiting, cache access or upstream work. Unlike
+  the cost-bearing routes it still answers same-origin requests that arrive
+  through a reverse proxy, so terrain keeps rendering behind nginx, Caddy or
+  Cloudflare instead of falling back to the geoid (Sunil56224972, daikaginza,
+  #975).