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>

.env.example

@@ -6,6 +6,12 @@ DATABASE_URL=postgresql://lyra:lyra@db:5432/lyra
 # 32 random bytes, base64. Generate with: openssl rand -base64 32
 LYRA_SECRET_KEY=MDEyMzQ1Njc4OWFiY2RlZjAxMjM0NTY3ODlhYmNkZWY=
 
+# Shared password protecting the whole web UI + API. When set, every request must sign in
+# (one shared password, no per-user accounts). LEAVE UNSET AND THE APP IS OPEN to anyone
+# who can reach the host — only safe on a trusted LAN or behind a VPN/auth proxy. Strongly
+# recommended for any exposed deployment. See README "Security".
+# LYRA_PASSWORD=change-me
+
 # Host directory for the downloaded music library (bind-mounted into the worker at /music)
 MUSIC_DIR=./music