fix(backend): better notes/translate error response (#13631)

* fix(backend): better `notes/translate` error response

* Update CHANGELOG.md

* test(backend): perform administrative operations as `root`

---------

Co-authored-by: syuilo <4439005+syuilo@users.noreply.github.com>

packages/backend/src/server/api/endpoints/notes/translate.ts

@@ -21,7 +21,7 @@ export const meta = {
 
 	res: {
 		type: 'object',
-		optional: false, nullable: false,
+		optional: true, nullable: false,
 		properties: {
 			sourceLang: { type: 'string' },
 			text: { type: 'string' },
@@ -39,6 +39,11 @@ export const meta = {
 			code: 'NO_SUCH_NOTE',
 			id: 'bea9b03f-36e0-49c5-a4db-627a029f8971',
 		},
+		cannotTranslateInvisibleNote: {
+			message: 'Cannot translate invisible note.',
+			code: 'CANNOT_TRANSLATE_INVISIBLE_NOTE',
+			id: 'ea29f2ca-c368-43b3-aaf1-5ac3e74bbe5d',
+		},
 	},
 } as const;
 
@@ -72,17 +77,17 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 			});
 
 			if (!(await this.noteEntityService.isVisibleForMe(note, me.id))) {
-				return 204; // TODO: 良い感じのエラー返す
+				throw new ApiError(meta.errors.cannotTranslateInvisibleNote);
 			}
 
 			if (note.text == null) {
-				return 204;
+				return;
 			}
 
 			const instance = await this.metaService.fetch();
 
 			if (instance.deeplAuthKey == null) {
-				return 204; // TODO: 良い感じのエラー返す
+				throw new ApiError(meta.errors.unavailable);
 			}
 
 			let targetLang = ps.targetLang;