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/docs.go

@@ -6286,10 +6286,15 @@ const docTemplate = `{
         "user.createUserRequest": {
             "type": "object",
             "required": [
+                "email",
                 "password",
                 "username"
             ],
             "properties": {
+                "email": {
+                    "type": "string",
+                    "maxLength": 255
+                },
                 "is_active": {
                     "type": "boolean"
                 },