feat(music): add Kugou (酷狗音乐) as a music source (#69)

Adds a self-contained Kugou provider (bilibili-style: direct API calls, no
embedded API server, no new npm dependency) plus full backend + WebUI wiring.

Provider (src/music/kugou.ts): search, song-url (with device registration),
lyrics (KRC decode), song detail, playlist, album, personal FM, QR login +
cookie persistence, and quality. Request signing / crypto / KRC decoding are
ported from the MIT-licensed MakcRe/KuGouMusicApi using Node's built-in
crypto and zlib (no third-party crypto packages).

Wiring: the "kugou" platform is threaded through the provider contract, queue,
play-history, bot instance/manager dispatch (getProviderFor + the -k command
flag), index/server composition, the music/player/auth routers (unified
/search/all, /quality, the platform coercions, QR login), the cookie store,
and the WebUI (search source tab + badge, SongCard badge, brand token, and a
Kugou QR/cookie login card in Settings).

Verified live during development: search, lyrics, and album playback resolve
correctly. NOT verifiable in CI (Kugou anti-bot blocks the build host's IP):
play-URL resolution, QR login, and VIP audio — these are built faithfully to
the reference and need end-to-end testing on a non-flagged IP / a Kugou
account. See the header comment in kugou.ts.

Includes src/music/kugou.test.ts (mappers + KRC→LRC). An adversarial review
pass fixed: pagination truncating on filtered counts, an ms/seconds duration
heuristic, dfid soft-fail caching, the /v5/url random-dfid fallback, the FM
body identity, and an unguarded nickname decode.

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

src/web/api/auth.ts

@@ -11,7 +11,8 @@ export function createAuthRouter(
   qqProvider: MusicProvider,
   bilibiliProvider: MusicProvider,
   logger: Logger,
-  cookieStore?: CookieStore
+  cookieStore?: CookieStore,
+  kugouProvider?: MusicProvider
 ): Router {
   const router = Router();
   // YouTube is auth-less; we only use this instance so /auth/status can
@@ -21,6 +22,7 @@ export function createAuthRouter(
   function getProvider(platform?: string): MusicProvider {
     if (platform === "bilibili") return bilibiliProvider;
     if (platform === "youtube") return youtubeProvider;
+    if (platform === "kugou" && kugouProvider) return kugouProvider;
     return platform === "qq" ? qqProvider : neteaseProvider;
   }
 
@@ -65,6 +67,7 @@ export function createAuthRouter(
       if (status === "confirmed") {
         const cookie = provider.getCookie();
         const plat = (platform as string) === "bilibili" ? "bilibili" as const
+          : (platform as string) === "kugou" ? "kugou" as const
           : (platform as string) === "qq" ? "qq" as const : "netease" as const;
         if (cookie && cookieStore) {
           cookieStore.save(plat, cookie);
@@ -137,6 +140,7 @@ export function createAuthRouter(
     const provider = getProvider(platform);
     provider.setCookie(cookie);
     const plat = platform === "bilibili" ? "bilibili" as const
+      : platform === "kugou" ? "kugou" as const
       : platform === "qq" ? "qq" as const : "netease" as const;
     if (cookieStore) {
       cookieStore.save(plat, cookie);