Fix NFS configuration: Remove ZFS mount point conflict with tmpfiles

- Remove /mnt/storage/media from systemd.tmpfiles.rules (it's a ZFS dataset mount point)
- Add ExecStartPost to set proper permissions on ZFS-mounted media directory
- Update NFS research documentation with ZFS integration best practices
- Add section explaining ZFS mount point vs tmpfiles.rules conflicts

This resolves the potential conflict where tmpfiles tries to create a directory
that ZFS wants to use as a mount point for the storage/media dataset.

modules/users/media-group.nix

@@ -9,4 +9,13 @@
   users.groups.media = {
     gid = 993; # Fixed GID for consistency across machines
   };
+
+  # Create media user for NFS anonymous mapping
+  users.users.media = {
+    uid = 993; # Fixed UID matching GID for NFS squashing
+    group = "media";
+    isSystemUser = true;
+    description = "Media files user for NFS squashing";
+    shell = "/run/current-system/sw/bin/nologin";
+  };
 }