single-user login screen

- server/credentials.py: PBKDF2 password + session secret, 0600 json,
  one-time setup command (server refuses to start without it)
- app.py: signed 30-day cookie (itsdangerous), middleware 401s API /
  redirects pages to /login?next=, 5-fail/15-min lockout, /logout
- static/login.html: dark-themed form, inline error display
- app.js: api() 401 -> redirect to /login preserving current URL
- docs: README quick start + features, SPEC auth section

requirements.txt

@@ -1,3 +1,4 @@
 fastapi
 uvicorn
 curl_cffi
+itsdangerous