music: 加 LLM chat、笔记 tab 化、歌单/标签

chat（右边栏）:
- chat_messages 表 per piece，OpenAI 兼容 /v1/chat/completions stream:true
- backend SSE forward delta，结束时落库 user + assistant
- system prompt 注入曲目 (title/artist/category/notes/lyrics 截 4KB)
- 网关同 mochi/config.yaml: gemma-4-31b-it on 3.135.65.204:8848，token 走 k8s Secret chat-creds
- reqwest client 去掉全局 timeout（chat 流可能跑很久），chord sidecar 调用改 per-request timeout

笔记: 从右 sidebar 移到独立 tab "笔记"

歌单 + tag:
- playlists / playlist_pieces / tags / piece_tags 表，CRUD API
- PATCH piece 接 tags 数组（按名字 upsert）
- list pieces 加 ?tag/?playlist 过滤 + 返回 tags 列表
- 顶 bar filterbar：歌单 + 标签 chip 切换；"+ 新歌单" prompt 创建
- EditView 加 tag 编辑（chip + 自动补全）+ 加入/移除歌单

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

@@ -8,8 +8,12 @@ async function jsonOrThrow(res) {
   return res.json()
 }
 
-export function listPieces() {
-  return fetch('/api/pieces').then(jsonOrThrow)
+export function listPieces({ tag, playlist } = {}) {
+  const qs = new URLSearchParams()
+  if (tag) qs.set('tag', tag)
+  if (playlist) qs.set('playlist', String(playlist))
+  const q = qs.toString()
+  return fetch('/api/pieces' + (q ? '?' + q : '')).then(jsonOrThrow)
 }
 
 export function getPiece(id) {
@@ -65,3 +69,126 @@ export function chordFetch(pieceId) {
 export function chordStatus(pieceId) {
   return fetch(`/api/pieces/${pieceId}/chord/status`).then(jsonOrThrow)
 }
+
+// ---- chat ----
+
+export function listChat(pieceId) {
+  return fetch(`/api/pieces/${pieceId}/chat`).then(jsonOrThrow)
+}
+
+export function clearChat(pieceId) {
+  return fetch(`/api/pieces/${pieceId}/chat`, { method: 'DELETE' }).then(jsonOrThrow)
+}
+
+/**
+ * stream a chat reply.
+ * @param {number} pieceId
+ * @param {string} message
+ * @param {(delta: string) => void} onDelta
+ * @param {AbortSignal} signal
+ * @returns {Promise<{ok: boolean, error?: string}>}
+ */
+export async function streamChat(pieceId, message, onDelta, signal) {
+  const resp = await fetch(`/api/pieces/${pieceId}/chat`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ message }),
+    signal,
+  })
+  if (!resp.ok) {
+    const text = await resp.text().catch(() => '')
+    return { ok: false, error: text || `${resp.status}` }
+  }
+  const reader = resp.body.getReader()
+  const dec = new TextDecoder()
+  let buf = ''
+  let lastEvent = 'message'
+  let errorMsg = ''
+  while (true) {
+    const { done, value } = await reader.read()
+    if (done) break
+    buf += dec.decode(value, { stream: true })
+    let idx
+    while ((idx = buf.indexOf('\n')) >= 0) {
+      const line = buf.slice(0, idx)
+      buf = buf.slice(idx + 1)
+      // SSE 协议：空行 = event 边界（已在内容中处理）；event:/data: 各自一行
+      if (line.startsWith('event:')) {
+        lastEvent = line.slice(6).trim()
+      } else if (line.startsWith('data:')) {
+        const data = line.slice(5).replace(/^ /, '')
+        if (lastEvent === 'error') {
+          errorMsg = data
+        } else if (lastEvent === 'done') {
+          // 不带 data 也算结束
+        } else {
+          onDelta(data)
+        }
+      } else if (line === '') {
+        lastEvent = 'message'
+      }
+    }
+  }
+  if (errorMsg) return { ok: false, error: errorMsg }
+  return { ok: true }
+}
+
+// ---- tags ----
+
+export function listTags() {
+  return fetch('/api/tags').then(jsonOrThrow)
+}
+
+export function createTag(name) {
+  return fetch('/api/tags', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ name }),
+  }).then(jsonOrThrow)
+}
+
+export function deleteTag(id) {
+  return fetch(`/api/tags/${id}`, { method: 'DELETE' }).then(jsonOrThrow)
+}
+
+// ---- playlists ----
+
+export function listPlaylists() {
+  return fetch('/api/playlists').then(jsonOrThrow)
+}
+
+export function createPlaylist(name, description) {
+  return fetch('/api/playlists', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ name, description: description || null }),
+  }).then(jsonOrThrow)
+}
+
+export function getPlaylist(id) {
+  return fetch(`/api/playlists/${id}`).then(jsonOrThrow)
+}
+
+export function patchPlaylist(id, body) {
+  return fetch(`/api/playlists/${id}`, {
+    method: 'PATCH',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  }).then(jsonOrThrow)
+}
+
+export function deletePlaylist(id) {
+  return fetch(`/api/playlists/${id}`, { method: 'DELETE' }).then(jsonOrThrow)
+}
+
+export function playlistAddPiece(playlistId, pieceId) {
+  return fetch(`/api/playlists/${playlistId}/pieces`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ piece_id: pieceId }),
+  }).then(jsonOrThrow)
+}
+
+export function playlistRemovePiece(playlistId, pieceId) {
+  return fetch(`/api/playlists/${playlistId}/pieces/${pieceId}`, { method: 'DELETE' }).then(jsonOrThrow)
+}