#42 [Admin Panel] Role Assignment API - Assign Roles

Implement POST /api/v1/admin/users/<user_id>/roles endpoint to assign roles to users. **API Specification:** - Endpoint: POST /api/v1/admin/users/<user_id>/roles - Permission Required: manage_users - Path Parameter: user_id (UUID) - Request Body: {roles: ['role_name1', 'role_name2']} - Response: {user_id, roles: updated_roles[]} **Requirements:** - Validate all role names exist - Idempotent - assigning existing role is no-op - Support bulk role assignment - Prevent removing last tenant_admin role if user is only admin - Log audit event with roles added **Validation:** - Role must exist in system - User must belong to current tenant - Cannot assign super_admin role via API **IMPORTANT:** Implement with ultra attention to details and correctness. **BEFORE CLOSING:** Take screenshot showing role assignment. Add comment summarizing implementation.