refactor and perf tweak

packages/backend/src/core/ReactionService.ts

@@ -30,6 +30,7 @@ import { RoleService } from '@/core/RoleService.js';
 import { FeaturedService } from '@/core/FeaturedService.js';
 
 const FALLBACK = '❤';
+const PER_NOTE_REACTION_USER_PAIR_CACHE_MAX = 16;
 
 const legacies: Record<string, string> = {
 	'like': '👍',
@@ -187,7 +188,7 @@ export class ReactionService {
 		await this.notesRepository.createQueryBuilder().update()
 			.set({
 				reactions: () => sql,
-				...(note.reactionAndUserPairCache.length < 10 ? {
+				...(note.reactionAndUserPairCache.length < PER_NOTE_REACTION_USER_PAIR_CACHE_MAX ? {
 					reactionAndUserPairCache: () => `array_append("reactionAndUserPairCache", '${user.id}/${reaction}')`,
 				} : {}),
 			})