Motivation for this change

Emby is now proprietary (see #51832), Jellyfin is an active fork and already in nixpkgs unstable since PR #57046.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

As for the implementation, this is a simplified copy of the Emby module:

• The pre-start script was removed in favor of specifying StateDirectory (at the cost of removing the dataDir option)
• Also specify the cache directory
• Add Jellyfin UID and GID

Notes:

I would have liked to use DynamicUser to avoid adding another UID and GID (we are dangerously approaching 399!), but since in a typical media server usecase, Jellyfin will have to access external directories, probably from another user, a static user and group is the simplest solution.

Another solution would have been to use DynamicUser, and add an option to specify extra static groups to add to the Jellyfin service (SupplementaryGroups), and let the user manage their media libraries accordingly.

One more minor detail is that I think the ffmpeg and ffprobe options now take two dashes instead of one and it isn't currently reflected in the jellyfin package. I don't mind changing that in this PR but it seemed somewhat out of place ^^