Add NFS self-heal automation for immich/calibre/ocis, backed up in git

Stale NFS handles inside containers (ESTALE/errno -116) can persist even
after the host-side mount looks healthy, requiring a manual remount +
container restart. Adds systemd mount-ready hooks (BindsTo) to restart
immich_server/calibre/ocis when their NFS mount unit restarts, plus a
5-minute health-check timer that does a real nested read and force-remounts
on staleness. Host files backed up under system-config/nfs-self-heal/ so
they can be reinstalled after a host rebuild.

system-config/nfs-self-heal/calibre-mount-ready.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=Restart calibre after NAS books mount is ready
+After=mnt-nas_books.mount
+Requires=mnt-nas_books.mount
+BindsTo=mnt-nas_books.mount
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/docker restart calibre
+RemainAfterExit=yes
+
+[Install]
+WantedBy=mnt-nas_books.mount