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/sma.nix

@@ -6,6 +6,10 @@
   pkgs,
   ...
 }: {
+  imports = [
+    ./media-group.nix
+  ];
+
   users.users.sma = {
     description = "Diziet Sma - System Administrator";
     isNormalUser = true;
@@ -20,6 +24,7 @@
       "incus-admin" # container management
       "podman" # container runtime
       "docker" # docker compatibility (if needed)
+      "media" # shared media access for NFS shares
     ];
 
     # Security-focused shell setup