Add multi-user support with admin user management

- Rewrite auth.py for multi-user schema (users table with roles)
- Auto-migrate from single-user admin_user table to new schema
- Add @admin_required decorator for protected routes
- Admin routes: /admin/users, /admin/users/new, delete, reset-password
- New templates: admin/users.html, user_new.html, user_created.html, password_reset.html
- Update login.html for username field, base.html and account.html for admin nav
- Max 16 users + 1 admin, session invalidation on delete/password reset

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontends/web/templates/base.html

@@ -46,6 +46,9 @@
                             </a>
                             <ul class="dropdown-menu dropdown-menu-end dropdown-menu-dark">
                                 <li><a class="dropdown-item" href="/account"><i class="bi bi-gear me-2"></i>Account</a></li>
+                                {% if is_admin %}
+                                <li><a class="dropdown-item" href="/admin/users"><i class="bi bi-people me-2"></i>Users</a></li>
+                                {% endif %}
                                 <li><hr class="dropdown-divider"></li>
                                 <li><a class="dropdown-item" href="/logout"><i class="bi bi-box-arrow-left me-2"></i>Logout</a></li>
                             </ul>