fix(server): gate cost/log endpoints against cross-site requests and ship a full CSP

The cost-bearing endpoints (/api/realtime/token, /api/openai/hud-summary,
/api/google/nearby-places) and the voice debug log refuse cross-site browser
requests: a foreign or opaque Origin, a Sec-Fetch-Site other than
same-origin/none, or proxy forwarding headers. Loopback tools without an
Origin and the LAN opt-in keep working. The dev and preview servers send one
shared Content-Security-Policy, and embed-mode framing rewrites only the
frame-ancestors directive of it.

Salvaged from #242 onto current main.

Co-authored-by: Sameh Khamis <[email protected]>
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

server/providers/common/same-site.js

@@ -0,0 +1,43 @@
+import { admitSameSiteRequest } from '../../../src/localRequestGate.mjs';
+
+/**
+ * Cross-site request gate for cost-bearing and log endpoints. Feeds the
+ * request to the pure, unit-tested `admitSameSiteRequest` and answers 403
+ * itself when the request is refused. Returns true when it has already
+ * responded (the caller returns); false when the handler should continue.
+ * Refuses cross-site browser requests (foreign or opaque Origin, a
+ * Sec-Fetch-Site other than same-origin/none, or reverse-proxy headers) while
+ * keeping loopback non-browser tools and the HOST=0.0.0.0 LAN opt-in working.
+ * @param {import('node:http').IncomingMessage} req
+ * @param {import('node:http').ServerResponse} res
+ * @returns {boolean} True when a 403 was sent.
+ */
+export function admitSameSite(req, res) {
+  const verdict = admitSameSiteRequest({
+    hostHeader: req.headers?.host,
+    protocol: req.socket?.encrypted ? 'https:' : 'http:',
+    origin: req.headers?.origin,
+    secFetchSite: req.headers?.['sec-fetch-site'],
+    proxyHeaders: req.headers || {},
+  });
+  if (verdict.ok) return false;
+  res.statusCode = verdict.status;
+  res.setHeader('Content-Type', 'application/json');
+  res.setHeader('Cache-Control', 'no-store');
+  res.end(JSON.stringify({ error: verdict.error }));
+  return true;
+}
+
+/**
+ * Wrap a middleware handler so cross-site requests are refused with 403
+ * before the handler runs. Loopback tools and same-origin browser requests
+ * pass straight through.
+ * @param {(req: import('node:http').IncomingMessage, res: import('node:http').ServerResponse, next?: Function) => unknown} handler
+ * @returns {(req: import('node:http').IncomingMessage, res: import('node:http').ServerResponse, next?: Function) => unknown}
+ */
+export function sameSiteGated(handler) {
+  return (req, res, next) => {
+    if (admitSameSite(req, res)) return undefined;
+    return handler(req, res, next);
+  };
+}