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/bot/instance.ts

@@ -36,6 +36,7 @@ export interface BotInstanceOptions {
   bilibiliProvider: MusicProvider;
   youtubeProvider: MusicProvider;
   localProvider?: MusicProvider;
+  kugouProvider?: MusicProvider;
   database: BotDatabase;
   config: BotConfig;
   logger: Logger;
@@ -69,6 +70,7 @@ export class BotInstance extends EventEmitter {
   private bilibiliProvider: MusicProvider;
   private youtubeProvider: MusicProvider;
   private localProvider: MusicProvider;
+  private kugouProvider: MusicProvider;
   private database: BotDatabase;
   private config: BotConfig;
   private logger: Logger;
@@ -98,6 +100,7 @@ export class BotInstance extends EventEmitter {
     this.bilibiliProvider = options.bilibiliProvider;
     this.youtubeProvider = options.youtubeProvider;
     this.localProvider = options.localProvider ?? options.neteaseProvider;
+    this.kugouProvider = options.kugouProvider ?? options.neteaseProvider;
     this.database = options.database;
     this.config = options.config;
     this.logger = options.logger.child({ botId: this.id });
@@ -524,10 +527,11 @@ export class BotInstance extends EventEmitter {
     }
   }
 
-  getProviderFor(platform: "netease" | "qq" | "bilibili" | "youtube" | "local"): MusicProvider {
+  getProviderFor(platform: "netease" | "qq" | "bilibili" | "youtube" | "local" | "kugou"): MusicProvider {
     if (platform === "bilibili") return this.bilibiliProvider;
     if (platform === "youtube") return this.youtubeProvider;
     if (platform === "local") return this.localProvider;
+    if (platform === "kugou") return this.kugouProvider;
     return platform === "qq" ? this.qqProvider : this.neteaseProvider;
   }
 
@@ -540,6 +544,7 @@ export class BotInstance extends EventEmitter {
     if (flags.has("b")) return this.bilibiliProvider;
     if (flags.has("q")) return this.qqProvider;
     if (flags.has("y")) return this.youtubeProvider;
+    if (flags.has("k")) return this.kugouProvider;
     return this.neteaseProvider;
   }