feat(phase3a): 음악 트랙 저장·조회 API (user_id + RLS)

- POST: 로그인 필수, createAdminClient로 music_tracks insert
- GET: 세션 클라이언트로 본인 것만(RLS music_select_own) 조회, 최신순
- JSON 파싱 try/catch(400), 필드 검증(str 헬퍼)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/api/studio/tracks/route.ts

@@ -0,0 +1,49 @@
+import { NextResponse } from 'next/server';
+import { createClient } from '@/lib/supabase/server';
+import { createAdminClient } from '@/lib/supabase/admin';
+
+export const runtime = 'nodejs';
+
+export async function POST(request: Request) {
+  const supabase = await createClient();
+  const { data: { user } } = await supabase.auth.getUser();
+  if (!user) return NextResponse.json({ error: '로그인이 필요합니다.' }, { status: 401 });
+
+  let body: Record<string, unknown>;
+  try {
+    body = await request.json();
+  } catch {
+    return NextResponse.json({ error: '잘못된 요청 형식' }, { status: 400 });
+  }
+
+  const str = (k: string) => (typeof body[k] === 'string' ? (body[k] as string) : null);
+
+  const admin = createAdminClient();
+  const { data, error } = await admin.from('music_tracks').insert({
+    user_id: user.id,
+    title: str('title'),
+    story: str('story'),
+    lyrics: str('lyrics'),
+    style: str('style'),
+    audio_url: str('audio_url'),
+    task_id: str('task_id'),
+  }).select('id, created_at').single();
+
+  if (error) return NextResponse.json({ error: error.message }, { status: 500 });
+  return NextResponse.json(data);
+}
+
+export async function GET() {
+  const supabase = await createClient();
+  const { data: { user } } = await supabase.auth.getUser();
+  if (!user) return NextResponse.json({ error: '로그인이 필요합니다.' }, { status: 401 });
+
+  // 세션 클라이언트로 본인 것만(RLS music_select_own)
+  const { data, error } = await supabase
+    .from('music_tracks')
+    .select('id, title, story, lyrics, style, audio_url, task_id, created_at')
+    .order('created_at', { ascending: false });
+
+  if (error) return NextResponse.json({ error: error.message }, { status: 500 });
+  return NextResponse.json({ tracks: data ?? [] });
+}