Support separate admin credentials for subsonic library scans

[isic96]

Hey there, great project I modified it slightly for my usecase and thought this might also be useful for others.
Details below:

Summary

• Adds optional ADMIN_SYSTEM_USERNAME and ADMIN_SYSTEM_PASSWORD config options
• When provided, admin credentials are used to trigger library scans while playlists are created under the regular user account

Use case

Subsonic-compatible servers like Navidrome require admin privileges to trigger library scans. This allows non-admin users to have personalized playlists without needing admin access to the media server.
My personal case is that I manage a Navidrome server for my family and I wanted to seutpo explo for each user to get their own playlist without all the accounts having admin rights.

Changes

• config.go: Added AdminCredentials struct
• subsonic.go: RefreshLibrary() uses admin credentials for scans if provided
• sample.env: Documented new options

[LumePart]

Thanks for the PR!