Fix Remux Dolby Vision MKVs task with Jellyfin 10.11 by avoiding GetUserByName

[MrHumanRebel]

Summary

This PR fixes the Remux Dolby Vision MKVs scheduled task failing on Jellyfin 10.11.x with a MissingMethodException:

System.MissingMethodException: Method not found: 'Jellyfin.Data.Entities.User MediaBrowser.Controller.Library.IUserManager.GetUserByName(System.String)'.

Jellyfin 10.11 removed or changed IUserManager.GetUserByName(string), so the plugin crashes as soon as the scheduled task runs.

Fixes #12.

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Changes

• Update RemuxLibraryTask to resolve the configured Primary User without calling the removed IUserManager.GetUserByName(string) method.
• Instead, the task now:
  • Reads configuration.PrimaryUser,
  • Trims it and performs a case-insensitive lookup against _userManager.Users by Name,
  • Throws a clear exception if the configured user does not exist (same behavior as before, just via a different API).

The rest of the task logic (item query, remux conditions, FFmpeg invocation, etc.) is unchanged.

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[katiekloss]

Thanks for the updates!

I think the "missing method" error is a bit misleading, because the method wasn't actually removed. The most recent version of the plugin was built for Jellyfin 10.10, and the IUserManager interface changed just enough in 10.11 that .NET isn't able to bind the GetUserByName method call to a target in the new version of the assembly.

(the order of declared methods might have changed, I think that can affect how the runtime picks targets.)

Rebuilding against 10.11.0 and .NET 9 appears to resolve the problem without any other code changes (though I have yet to test it), so I'm going to close this for now.