fix(server): keep debug-log timestamps server-owned and send nosniff

The voice debug log writes the server's timestamp after the client's record,
so a record can no longer replace loggedAt. The dev and preview servers also
send X-Content-Type-Options: nosniff with their shared browser headers.

From the findings in #710.

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

src/tooling/localServices.test.mjs

@@ -391,8 +391,8 @@ test('the debug-log sink stays bounded, rate limited, and quiet about failures',
     error: 'Failed to write Realtime debug log',
   });
 
-  // The limiter is always on, unlike the opt-in one the cost-bearing routes
-  // share: 120/min per IP, far above what a voice session writes.
+  // This limiter has no opt-out, unlike the one the cost-bearing routes share:
+  // 120/min per IP, far above what a voice session writes.
   let limited = null;
   let accepted = 0;
   for (let n = 0; n < 130 && !limited; n += 1) {
@@ -415,6 +415,24 @@ test('the debug-log sink stays bounded, rate limited, and quiet about failures',
   assert.ok(lines.every((line) => JSON.parse(line).loggedAt));
 });
 
+test('a debug-log record cannot supply its own timestamp', async (t) => {
+  const sourceRoot = root(t);
+  const handler = install(openAiRealtimeProxy({ sourceRoot })).get(
+    '/api/realtime/debug-log',
+  );
+  const before = Date.now();
+  const response = await request(handler, {
+    method: 'POST',
+    body: JSON.stringify({ loggedAt: '1999-01-01T00:00:00.000Z', note: 'x' }),
+  });
+  assert.equal(response.status, 204);
+  const file = path.join(sourceRoot, '.gev-logs/realtime-conversations.jsonl');
+  const [line] = readFileSync(file, 'utf8').split('\n').filter(Boolean);
+  const record = JSON.parse(line);
+  assert.equal(record.note, 'x');
+  assert.ok(Date.parse(record.loggedAt) >= before - 1000);
+});
+
 test('an oversized debug-log request receives the fixed error response', async (t) => {
   const handler = install(openAiRealtimeProxy({ sourceRoot: root(t) })).get(
     '/api/realtime/debug-log',

src/tooling/viteBuild.test.mjs

@@ -19,6 +19,7 @@ test('explicit build inputs preserve browser-only defines, plugin order and loop
   assert.ok(config.server.fs.deny.includes('**/ENVIRONMENT'));
   assert.ok(config.server.fs.deny.includes('.env.*'));
   assert.equal(config.server.headers['X-Frame-Options'], 'DENY');
+  assert.equal(config.server.headers['X-Content-Type-Options'], 'nosniff');
   const csp = config.server.headers['Content-Security-Policy'];
   assert.equal(csp, BROWSER_CSP);
   for (const directive of [