Nhạc Kèn Công Giáo API — OK

Endpoints:
  GET /api/songs-index          → list all songs (metadata)
  GET /api/song/:id             → song details + arrangements
  GET /api/association/:id      → association detail
  GET /api/associations         → list all associations
  GET /pdf/:fileId              → Google Drive PDF proxy

  GET /api/sax-songs            → list all sax songs
  POST /unlock                  → mở khoá bài (id + pass)
Firebase: nhackenconggiao-78784
Cache: 300s API / 3600s PDF