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/media-group.nix

@@ -0,0 +1,12 @@
+# Media Group Configuration
+# Shared group for NFS media access permissions
+{
+  config,
+  pkgs,
+  ...
+}: {
+  # Create the media group for shared NFS access
+  users.groups.media = {
+    gid = 993; # Fixed GID for consistency across machines
+  };
+}