Add API root endpoint for /api/users/

Added UsersAPIRoot view that returns available user-related
endpoints when accessing /api/users/ directly.

All API endpoint groups now have discoverable root endpoints:
- /api/ - Main API root
- /api/users/ - User endpoints
- /api/tasks/ - Task endpoints (already had TaskListCreateAPIView)
- /api/sync/ - Sync endpoints (already had sync view)
- /api/notifications/ - Notification endpoints (already had NotificationListAPIView)

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

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

users/urls.py

@@ -9,6 +9,7 @@ from . import views
 
 # API URLs (to be included under /api/users/)
 api_urlpatterns = [
+    path('', views.UsersAPIRoot.as_view(), name='api-users-root'),
     path('register/', views.RegisterAPIView.as_view(), name='api-register'),
     path('verify-email/', views.VerifyEmailAPIView.as_view(), name='api-verify-email'),
     path('resend-verification/', views.ResendVerificationEmailAPIView.as_view(), name='api-resend-verification'),