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/login.html

@@ -17,7 +17,7 @@
                             <i class="bi bi-person me-1"></i> Username
                         </label>
                         <input type="text" name="username" class="form-control"
-                               value="{{ username }}" readonly>
+                               placeholder="Enter your username" required autofocus>
                     </div>
 
                     <div class="mb-4">
@@ -26,7 +26,7 @@
                         </label>
                         <div class="input-group">
                             <input type="password" name="password" class="form-control"
-                                   id="passwordInput" required autofocus>
+                                   id="passwordInput" required>
                             <button class="btn btn-outline-secondary" type="button"
                                     onclick="togglePassword('passwordInput', this)">
                                 <i class="bi bi-eye"></i>