feat: 曲库索引 —— 曲库里已有的同名歌曲在列表中标记「曲库」

不是通过声匣下载的文件（其他工具 / 手动放入）以前一律显示未下载。
现在启动时扫描音乐目录，按「歌名 + 歌手文件夹 / 专辑文件夹」匹配，
下载完成增量更新，每 30 分钟或删除 / 补全标签后重建；设置页可手动重建。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/libraryIndex.ts

@@ -0,0 +1,108 @@
+// 曲库索引：扫描音乐目录，按「歌名 + 歌手文件夹 / 专辑文件夹」判断某首歌是否已在曲库中，
+// 用于给不是通过本服务下载的文件也打上标记。
+import fs from 'node:fs';
+import path from 'node:path';
+import { config } from './config.ts';
+import { sanitize, sanitizePathSegment } from './core/util.ts';
+
+const AUDIO_EXTS = new Set(['.mp3', '.flac', '.m4a', '.ogg', '.wav', '.aac', '.ape', '.opus']);
+const REBUILD_INTERVAL = 30 * 60 * 1000;
+
+interface Entry {
+  artists: Set<string>;
+  album: string;
+  file: string; // 相对音乐目录
+}
+
+/** 比较用的规范化：全角转半角、小写、去掉空白与引号类符号 */
+export function norm(s: string): string {
+  return s
+    .normalize('NFKC')
+    .toLowerCase()
+    .replace(/[\s'"’‘“”`·・]/g, '');
+}
+
+let index = new Map<string, Entry[]>();
+let building: Promise<void> | null = null;
+let builtAt = 0;
+let fileCount = 0;
+
+function addFile(map: Map<string, Entry[]>, rel: string) {
+  const parts = rel.split('/');
+  const fileName = parts.at(-1)!;
+  const base = fileName.slice(0, fileName.length - path.extname(fileName).length);
+  const artists = new Set<string>();
+  let title = base.replace(/^\d+[.\s_-]+\s*/, '');
+
+  // 「歌手 - 歌名」（旧版扁平格式）
+  const dash = base.match(/^(.+?)\s+-\s+(.+)$/);
+  if (dash && !/^\d+[.\s]/.test(base)) {
+    title = dash[2]!;
+    for (const a of dash[1]!.split(/\s*[/_&,，、]\s*/)) if (a) artists.add(norm(a));
+  }
+  // 「歌手/专辑/NN. 歌名」：第一层目录是歌手（专辑艺人）
+  if (parts.length >= 2) artists.add(norm(parts[0]!));
+  const album = parts.length >= 3 ? norm(parts.at(-2)!) : '';
+
+  const key = norm(title);
+  if (!key) return;
+  const list = map.get(key) ?? [];
+  list.push({ artists, album, file: rel });
+  map.set(key, list);
+}
+
+async function walk(dir: string, rel: string, map: Map<string, Entry[]>) {
+  const entries = await fs.promises.readdir(dir, { withFileTypes: true }).catch(() => []);
+  for (const e of entries) {
+    if (e.name.startsWith('.') || e.name.startsWith('@')) continue;
+    const childRel = rel ? `${rel}/${e.name}` : e.name;
+    if (e.isDirectory()) await walk(path.join(dir, e.name), childRel, map);
+    else if (AUDIO_EXTS.has(path.extname(e.name).toLowerCase())) {
+      addFile(map, childRel);
+      fileCount++;
+    }
+  }
+}
+
+export function rebuildLibraryIndex(): Promise<void> {
+  building ??= (async () => {
+    const started = Date.now();
+    const map = new Map<string, Entry[]>();
+    fileCount = 0;
+    await walk(config.musicDir, '', map);
+    index = map;
+    builtAt = Date.now();
+    console.log(`📚 曲库索引：${fileCount} 个音频文件，用时 ${Date.now() - started} ms`);
+  })().finally(() => (building = null));
+  return building;
+}
+
+/** 下载完成后增量加入，不必等下次重建 */
+export function addToLibraryIndex(relFile: string) {
+  addFile(index, relFile.split(path.sep).join('/'));
+}
+
+export function startLibraryIndex() {
+  void rebuildLibraryIndex();
+  setInterval(() => void rebuildLibraryIndex(), REBUILD_INTERVAL).unref();
+}
+
+export function libraryIndexInfo() {
+  return { files: fileCount, builtAt: builtAt ? new Date(builtAt).toISOString() : null, building: !!building };
+}
+
+/**
+ * 在曲库中查找同名歌曲：歌名一致，且歌手文件夹与任一歌手一致或专辑文件夹与专辑一致。
+ * 返回匹配到的文件（相对路径），找不到返回 undefined。
+ */
+export function findInLibrary(track: { name: string; artists: string; album?: string }): string | undefined {
+  // 下载时文件名经过 sanitize，这里同样处理，"A/B" 才能对上 "A_B"
+  const candidates = index.get(norm(sanitize(track.name)));
+  if (!candidates) return undefined;
+  const artists = track.artists
+    .split(/\s*\/\s*/)
+    .filter(Boolean)
+    .map((a) => norm(sanitizePathSegment(a, a)));
+  const album = track.album ? norm(sanitizePathSegment(track.album, track.album)) : '';
+  return candidates.find((c) => artists.some((a) => c.artists.has(a)) || (album && c.album === album))?.file;
+}