fix(backup): TLS client certificates persist in Docker and are backed up

libvirt reads a TLS compute's client certificate from the panel user's
~/.pki/libvirt. In Docker that was /var/www/.pki inside the container,
so `docker compose up -d --build` dropped it and every TLS compute
stopped connecting; no backup held it on either install type, although
"Moving to a new machine" says the backup carries what the computes
accept, and the README never said where the certificates go.

Docker keeps /var/www/.pki in a webvirtcloud-pki volume, owned by
www-data and private like ~/.ssh; the backup copies ~/.pki into pki/;
the moving steps put it back; and "Adding a Compute Node" says where the
files go.

docker-compose.yml

@@ -37,6 +37,8 @@ services:
       - webvirtcloud-data:/srv/webvirtcloud/data
       # Persistent SSH keys for compute node authentication
       - webvirtcloud-ssh:/var/www/.ssh
+      # Persistent TLS client certificates for TLS computes (.pki/libvirt)
+      - webvirtcloud-pki:/var/www/.pki
       # Daily backups of both (manage.py backup)
       - webvirtcloud-backups:/srv/webvirtcloud/backups
 
@@ -45,5 +47,7 @@ volumes:
     driver: local
   webvirtcloud-ssh:
     driver: local
+  webvirtcloud-pki:
+    driver: local
   webvirtcloud-backups:
     driver: local