Add secure username/password login portal

Gate the suite behind a self-contained login (no external IdP):

- User model with bcrypt-hashed passwords; admin/user roles
- /api/auth endpoints: login, logout, me, change-password, and
  admin-only user management (list/create/delete/reset/enable)
- Stateless JWT session in an HttpOnly, SameSite=Lax, auto-Secure
  cookie; middleware refuses every /api data route without a session
- login.html + auth-guard.js: login page and per-page guard with a
  top-right "name / Admin / Sign out" pill
- Admin Console now gated on admin role (passphrase gate removed) with
  a User administration card
- manage_users.py CLI to bootstrap the first admin
- Rebuilt help.js into a searchable, multi-topic help center
- Local-dev convenience: app serves html/ so the site + API share one
  origin under uvicorn (inactive in the prod container)
- Docs/env: AUTH_SECRET_KEY, requirements (bcrypt, PyJWT), README

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

html/index.html

@@ -4,6 +4,7 @@
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <title>Work Package Suite — Prime Controls</title>
+  <script src="auth-guard.js"></script>
   <link rel="icon" href="favicon.ico" sizes="any">
   <link rel="stylesheet" href="theme-light.css">
   <style>