Add NixOS configuration for john-endesktop ZFS/NFS server

Create configuration to migrate john-endesktop from Arch Linux to NixOS while
maintaining existing ZFS pools (media JBOD and swarmvols mirror) and NFS
exports for k3s cluster.

Configuration includes:
- ZFS support with automatic pool import
- NFS server exporting both pools to 10.0.0.0/24
- Correct ZFS hostid (007f0101) to resolve hostid warnings
- Btrfs root filesystem on nvme0n1p5 (810GB)
- Comprehensive migration plan with rollback procedures

The migration is designed to be safe with Arch Linux remaining bootable
as a fallback until NixOS is verified stable.

flake.nix

@@ -209,6 +209,15 @@
       ];
     };
 
+    # ZFS/NFS server configuration
+    nixosConfigurations.john-endesktop = nixpkgs.lib.nixosSystem rec {
+      system = "x86_64-linux";
+      modules = nixosModules ++ [
+        ./machines/john-endesktop/configuration.nix
+        # Minimal server - no home-manager needed
+      ];
+    };
+
     # Darwin/macOS configurations
     darwinConfigurations."blkfv4yf49kt7" = inputs.nix-darwin.lib.darwinSystem rec {
       system = "aarch64-darwin";