feat(flags): serve /api/flags, and keep the public proxy off it

The endpoint and its exposure control ship together on purpose. The
moment /api/flags exists, app/api/[...path] forwards it — and the
response names every unreleased feature the codebase knows about, along
with whether it is on. Publishing that is the opposite of shipping dark.

Denied on an exact first-segment match, not a prefix, so /api/flagship
does not go down with /api/flags. Next reads the endpoint server-side
over the Docker network, which never transits the public proxy.

jest.setup.js now guards its browser globals. It runs for every suite,
including the one that declares @jest-environment node to exercise the
route handler — NextRequest needs Fetch API globals jsdom lacks, and
there is no window there to define matchMedia on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

nextjs-app/__tests__/api/proxyDenylist.test.ts

@@ -0,0 +1,31 @@
+/**
+ * The /api/* proxy is public. Anything it forwards is on the internet.
+ *
+ * @jest-environment node
+ */
+// The docblock above is load-bearing. jest.config.js sets jsdom globally, and
+// NextRequest/NextResponse need the Web Fetch API globals that only the node
+// environment provides — under jsdom this suite fails on import, not on an
+// assertion.
+import { NextRequest } from 'next/server';
+import { GET } from '@/app/api/[...path]/route';
+
+function request(path: string) {
+  return new NextRequest(`http://localhost:3000/api/${path}`);
+}
+
+describe('public API proxy', () => {
+  it('refuses to forward internal-only paths', async () => {
+    // /api/flags names every unreleased feature and its state. Forwarding it
+    // publishes the thing shipping dark exists to keep quiet.
+    const res = await GET(request('flags'), { params: Promise.resolve({ path: ['flags'] }) });
+    expect(res.status).toBe(404);
+  });
+
+  it('does not deny a path that merely starts with the same letters', async () => {
+    // A prefix match would take /api/flagship down with /api/flags.
+    const res = await GET(
+      request('flagship'), { params: Promise.resolve({ path: ['flagship'] }) });
+    expect(res.status).not.toBe(404);
+  });
+});