music: 新建 music app，替换 piano-sheet

听歌 + 练琴曲目管理：
- 数据：piece (title/artist/category/lyrics/play_count/notes) + attachments (audio/video/pdf/image; image 带 role=chord/numbered/staff)
- 后端 axum + sqlite，附件流式落 PVC，ServeFile 支持 Range（视频拖动）
- 前端 guitar 风格 player：左 sidebar + tabs(歌词/吉他谱/简谱/五线谱/PDF/视频)，LRC 同步、快捷键、笔记自动保存
- ns cube-music + music.famzheng.me + bodylimit 5GiB
- scripts/import_guitar.py 用于把 oci /data/guitar/ 旧曲库导入

apps/music/frontend/src/lib/api.js

@@ -0,0 +1,59 @@
+// 薄薄一层 fetch 封装。错误统一抛 Error(message)。
+
+async function jsonOrThrow(res) {
+  if (!res.ok) {
+    const text = await res.text().catch(() => '')
+    throw new Error(text || `${res.status} ${res.statusText}`)
+  }
+  return res.json()
+}
+
+export function listPieces() {
+  return fetch('/api/pieces').then(jsonOrThrow)
+}
+
+export function getPiece(id) {
+  return fetch(`/api/pieces/${id}`).then(jsonOrThrow)
+}
+
+export function createPiece(body) {
+  return fetch('/api/pieces', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  }).then(jsonOrThrow)
+}
+
+export function patchPiece(id, body) {
+  return fetch(`/api/pieces/${id}`, {
+    method: 'PATCH',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  }).then(jsonOrThrow)
+}
+
+export function deletePiece(id) {
+  return fetch(`/api/pieces/${id}`, { method: 'DELETE' }).then(jsonOrThrow)
+}
+
+export function recordPlay(id) {
+  return fetch(`/api/pieces/${id}/play`, { method: 'POST' }).then(jsonOrThrow)
+}
+
+// `role`: null | 'chord' | 'numbered' | 'staff'
+export function uploadAttachments(pieceId, files, role) {
+  const fd = new FormData()
+  for (const f of files) fd.append('files', f, f.name)
+  const url = role
+    ? `/api/pieces/${pieceId}/attachments?role=${encodeURIComponent(role)}`
+    : `/api/pieces/${pieceId}/attachments`
+  return fetch(url, { method: 'POST', body: fd }).then(jsonOrThrow)
+}
+
+export function deleteAttachment(id) {
+  return fetch(`/api/attachments/${id}`, { method: 'DELETE' }).then(jsonOrThrow)
+}
+
+export function attachmentUrl(id) {
+  return `/api/attachments/${id}`
+}