清理qBittorrent接入残留：删除死schema分组与前端下载类型，下载器保存目录键改为downloader.savepath并兼容旧键

internal/service/crypto.go

@@ -1,6 +1,6 @@
 // Package service — AES-GCM crypto helper for at-rest secrets.
 //
-// Sensitive fields (third-party API keys, qBittorrent passwords, …) are
+// Sensitive fields (third-party API keys, service passwords, …) are
 // stored in SQLite. We encrypt them with AES-256-GCM keyed off the JWT
 // secret so a stolen DB file alone is not enough to recover the
 // plaintext credentials.