#899: map /app/MusicVideos in the Unraid template

The image declares /app/MusicVideos as a VOLUME but the template never mapped it,
so music-video downloads landed in an anonymous Docker volume. Add an (advanced,
optional) Music Videos path mount so users can point it at a share.

templates/soulsync.xml

@@ -29,6 +29,7 @@
   <Config Name="Database Volume" Target="/app/data" Default="/mnt/user/appdata/soulsync/data" Mode="rw" Description="Database storage (SQLite)" Type="Path" Display="advanced" Required="false" Mask="false">/mnt/user/appdata/soulsync/data</Config>
   <Config Name="Downloads" Target="/app/downloads" Default="/mnt/user/downloads/" Mode="rw" Description="Path to Soulseek (slskd) downloads folder — should match your slskd download path" Type="Path" Display="always" Required="false" Mask="false">/mnt/user/downloads/</Config>
   <Config Name="Library/Transfer" Target="/app/Transfer" Default="/mnt/user/library/" Mode="rw" Description="Your music library folder for organized/transferred files" Type="Path" Display="always" Required="false" Mask="false">/mnt/user/library/</Config>
+  <Config Name="Music Videos" Target="/app/MusicVideos" Default="/mnt/user/media/music-videos/" Mode="rw" Description="Optional — folder for downloaded music videos. Point this at a share if you use the music-video feature, otherwise downloads land in an anonymous Docker volume. Set library.music_videos_path in the app to /app/MusicVideos." Type="Path" Display="advanced" Required="false" Mask="false">/mnt/user/media/music-videos/</Config>
   <Config Name="PUID" Target="PUID" Default="99" Mode="" Description="User ID for file permissions (default 99 = nobody on Unraid)" Type="Variable" Display="always" Required="false" Mask="false">99</Config>
   <Config Name="PGID" Target="PGID" Default="100" Mode="" Description="Group ID for file permissions (default 100 = users on Unraid)" Type="Variable" Display="always" Required="false" Mask="false">100</Config>
   <Config Name="Timezone" Target="TZ" Default="America/New_York" Mode="" Description="Timezone for log timestamps and scheduling (e.g., America/New_York)" Type="Variable" Display="always" Required="false" Mask="false">America/New_York</Config>