feat(phase3a): ai-usage에 music 추가 + music_tracks·CHECK 마이그레이션

- lib/ai-usage.ts: MUSIC_DAILY_LIMIT=1 추가, AiService 타입에 'music' 포함
- lib/__tests__/ai-usage.test.ts: MUSIC_DAILY_LIMIT 상수 검증 테스트 추가
- supabase/migrations/2026-07-03-phase3a-music.sql: music_tracks 테이블, CHECK 확장, service_settings 정리

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AAtcmKKtqDUe4NyVgy1aLQ

lib/ai-usage.ts

@@ -2,7 +2,8 @@ import type { SupabaseClient } from '@supabase/supabase-js';
 
 export const SAJU_DAILY_LIMIT = 1;
 export const TAROT_DAILY_LIMIT = 3;
-export type AiService = 'saju' | 'tarot';
+export const MUSIC_DAILY_LIMIT = 1;
+export type AiService = 'saju' | 'tarot' | 'music';
 
 /** KST(UTC+9) 자정을 UTC ISO로. 오늘 사용량 집계 하한. */
 export function kstDayStartISO(now: Date): string {