fix: cidCache 大小限制为500条，防止内存泄漏

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/music/bilibili.ts

@@ -161,6 +161,12 @@ export class BiliBiliProvider implements MusicProvider {
     const cached = this.cidCache.get(bvid);
     if (cached) return cached;
 
+    // Limit cache size to prevent unbounded growth
+    if (this.cidCache.size > 500) {
+      const firstKey = this.cidCache.keys().next().value;
+      if (firstKey) this.cidCache.delete(firstKey);
+    }
+
     const detail = await this.getSongDetail(bvid);
     if (!detail) return null;
     return this.cidCache.get(bvid) ?? null;