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

supabase/migrations/2026-07-03-phase3a-music.sql

@@ -0,0 +1,23 @@
+-- Phase 3a (2026-07-03): 음악 회원 저장 + 사용량 로그 확장 + 음악 숨김 해제
+-- 의존성: 2026-07-02-phase2-saju-tarot.sql(ai_usage_log 생성) 적용 후 실행
+-- 적용: 클라우드 Supabase + NAS self-host 양쪽
+
+CREATE TABLE IF NOT EXISTS music_tracks (
+  id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id uuid NOT NULL REFERENCES auth.users(id) ON DELETE CASCADE,
+  title text,
+  story text,
+  lyrics text,
+  style text,
+  audio_url text,
+  task_id text,
+  created_at timestamptz NOT NULL DEFAULT now()
+);
+ALTER TABLE music_tracks ENABLE ROW LEVEL SECURITY;
+CREATE POLICY music_select_own ON music_tracks FOR SELECT USING (auth.uid() = user_id);
+
+-- ai_usage_log CHECK에 'music' 추가 (phase2의 인라인 CHECK auto-name 제거 후 재정의)
+ALTER TABLE ai_usage_log DROP CONSTRAINT IF EXISTS ai_usage_log_service_check;
+ALTER TABLE ai_usage_log ADD CONSTRAINT ai_usage_log_service_check CHECK (service IN ('saju','tarot','music'));
+
+DELETE FROM service_settings WHERE id = 'music';