tweak custom emoji handling

Close #9721

packages/backend/src/core/entities/NotificationEntityService.ts

@@ -146,6 +146,8 @@ export class NotificationEntityService implements OnModuleInit {
 			myReactionsMap.set(target, myReactions.find(reaction => reaction.noteId === target) ?? null);
 		}
 
+		await this.customEmojiService.prefetchEmojis(this.customEmojiService.aggregateNoteEmojis(notes));
+
 		return await Promise.all(notifications.map(x => this.pack(x, {
 			_hintForEachNotes_: {
 				myReactions: myReactionsMap,