Implement media group for NFS permission management

- Create shared media-group.nix module with fixed GID (993)
- Add both geir and sma users to media group for shared NFS access
- Update NFS server configuration to use root:media ownership with 0775 permissions
- Convert all media services to use media group instead of users group:
  - Jellyfin, Calibre-web, Audiobookshelf, Transmission
- Enable group write access to all NFS shares (/mnt/storage/*)
- Maintain security with root ownership while allowing group collaboration

This resolves NFS permission issues by providing consistent group-based access
control across all media services and storage directories.

modules/users/geir.nix

@@ -8,6 +8,10 @@
   # Import custom packages from the flake
   homeLabPackages = import ../../packages {inherit pkgs;};
 in {
+  imports = [
+    ./media-group.nix
+  ];
+
   users.users.geir = {
     description = "Geir Okkenhaug Jerstad - Primary User";
     isNormalUser = true;
@@ -22,6 +26,7 @@ in {
       "audio" # audio devices
       "video" # video devices
       "render" # GPU access
+      "media" # shared media access for NFS shares
     ];
 
     shell = pkgs.zsh;