feat(web): minimal shared-password auth gate

The web app had no auth — anyone reaching the host could browse the library,
trigger downloads, and read/overwrite the stored credentials in Settings. Adds
an opt-in single-shared-password gate:

- Next.js middleware redirects unauthenticated page requests to /login and
  returns 401 for /api/*; static assets are excluded via matcher.
- Auth is enabled ONLY when LYRA_PASSWORD is set (unset ⇒ app stays open, so
  existing deployments are unchanged). Documented in README "Security".
- The session cookie is httpOnly and carries HMAC-SHA256(password) keyed by
  LYRA_SECRET_KEY — unforgeable, and the plaintext password never touches the
  client. Edge-safe Web Crypto so one lib/auth.ts serves middleware + route.
- Cookie is NOT Secure: Lyra serves plain HTTP on the LAN; a Secure cookie would
  loop. Works behind an HTTPS proxy too.
- /login page + form, a Sign out button (shown only when auth is enabled), and
  chrome (nav + worker-status poll) suppressed on /login.
- docker-compose passes LYRA_PASSWORD to web; .env.example documents it.

Verified live end-to-end (built + next start): no-cookie page→307 /login,
API→401, wrong password→401, correct→200+cookie, authed page/API→200, forged
cookie→401, and the auth-disabled (no LYRA_PASSWORD) path stays fully open.
web 133 tests green, tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/_ui/sign-out.tsx

@@ -0,0 +1,20 @@
+"use client";
+
+import { usePathname } from "next/navigation";
+
+/** Clears the auth cookie and returns to /login. Rendered only when auth is enabled. */
+export function SignOut() {
+  const pathname = usePathname();
+  if (pathname === "/login") return null;
+
+  async function signOut() {
+    await fetch("/api/auth", { method: "DELETE" }).catch(() => {});
+    window.location.assign("/login");
+  }
+
+  return (
+    <button type="button" className="sign-out" onClick={signOut} aria-label="sign out">
+      Sign out
+    </button>
+  );
+}