feat: add Role (owner|admin) to the user domain, persistence model and mapper

features/user_response_field_hygiene.qa.md

@@ -21,11 +21,12 @@ Package: `test` (black-box), extending `test/account_verification_http_test.go`'
    user id 7. Decode the response body into `map[string]json.RawMessage` (not into `user.User`)
    and assert:
    - the key set is exactly `{"id", "username", "email", "is_verified", "created_at",
-     "updated_at", "display_name"}`;
+     "updated_at", "display_name", "role"}`;
    - none of `password`, `password_reset_code`, `verification_code`,
      `verification_expiration`, `password_reset_expiration`, `token_version` is present;
    - `is_verified` unmarshals to `true`;
-   - `display_name` unmarshals to `""`.
+   - `display_name` unmarshals to `""`;
+   - `role` unmarshals to `"owner"` (seeded user 7 has `Role: owner`).
 5. Rejected verify: same seed, `{"code":"wrong-code"}` → assert status `403` and that the raw
    response body bytes, treated as a string, contain none of the substrings `password`,
    `verification_code`, `password_reset_code`, `verification_expiration`,