feat: implement user management system

- Add role field to UserProfile (superadmin/admin/trainer)
- Add role-based permission classes
- Create UserManagementViewSet with CRUD and password change
- Add API types and components for user management
- Create users management page in settings
- Only superadmins can manage users

.env.example

@@ -0,0 +1,6 @@
+# Backend Environment
+SECRET_KEY=change-me-to-a-long-random-string-in-production
+DATABASE_URL=sqlite:///db.sqlite3
+
+# For PostgreSQL (recommended for production):
+# DATABASE_URL=postgresql://user:password@db:5432/wrestledesk