system-config: back up nas_audiobooks mount fix and missing unit files

Adds the 4 permanent .mount units (family/books/owncloud/audiobooks) and
audiobookshelf-mount-ready.service to the repo backup, which weren't
tracked here yet, and updates the README to document the autofs->permanent
mount fix applied to nas_audiobooks today so a host rebuild doesn't
recreate the broken automount version.

system-config/nfs-self-heal/nfs-mount-heal.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 # Deep health check + self-heal for permanent NFS mounts backing
-# immich, calibre, and ocis.
+# immich, calibre, ocis, and audiobookshelf.
 #
 # A plain `ls` on a mount root can succeed even when nested file
 # handles inside it are stale (ESTALE / errno -116) -- that's the
@@ -20,6 +20,7 @@ CHECKS=(
   "/mnt/nas_family:mnt-nas_family.mount:/mnt/nas_family/immich-library/thumbs:immich_server"
   "/mnt/nas_books:mnt-nas_books.mount:/mnt/nas_books/calibre-import:calibre"
   "/mnt/nas_owncloud:mnt-nas_owncloud.mount:/mnt/nas_owncloud:ocis"
+  "/mnt/nas_audiobooks:mnt-nas_audiobooks.mount:/mnt/nas_audiobooks:audiobookshelf"
 )
 
 alert() {