feat(user): require email when admin creates a user

- Add `email` as a required field in `createUserRequest`
- Enforce email format verification and database uniqueness checks in the admin user creation handler
- Update the admin user creation frontend modal with validation and form field
- Update the corresponding backend unit tests and regenerate Swagger docs

docs/swagger.yaml

@@ -987,6 +987,9 @@ definitions:
     type: object
   user.createUserRequest:
     properties:
+      email:
+        maxLength: 255
+        type: string
       is_active:
         type: boolean
       is_admin:
@@ -1003,6 +1006,7 @@ definitions:
         minLength: 3
         type: string
     required:
+    - email
     - password
     - username
     type: object