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/jest.setup.js

@@ -12,6 +12,12 @@ jest.mock('next/navigation', () => ({
   useSearchParams: () => new URLSearchParams(),
 }));
 
+// Everything below this line is browser furniture, and this file runs for
+// every suite — including the ones that declare `@jest-environment node` to
+// test route handlers, where NextRequest needs Fetch API globals jsdom does
+// not provide. There is no `window` there, so guard rather than assume one.
+if (typeof window !== 'undefined') {
+
 // Mock window.matchMedia
 Object.defineProperty(window, 'matchMedia', {
   writable: true,
@@ -52,3 +58,5 @@ const localStorageMock = {
   clear: jest.fn(),
 };
 global.localStorage = localStorageMock;
+
+} // end: browser-only globals