Create each Site's network with its own subnet

The deploy failed on hngmohali with 'all predefined address pools have been
fully subnetted': every 172.17-172.30/16 is in use, 172.31/16 is the host's own
VPC so Docker will not allocate from it, and all sixteen 192.168.0-240/20 blocks
are taken. Docker therefore refuses a network created without a subnet.

Each Site now names a free network_subnet (10.210.0.0/16, one /24 each), the role
looks the network up first and only creates it when it is absent and with that
subnet, and it fails with an explanation when a Site declares none. Existing
networks are left alone, because Docker cannot change a subnet in place and the
Sites already running have theirs. The test checks every Site declares one, and
the deployment readme says a new Site needs a free subnet.

deploy.sh now deploys every Site by default, with SITE=<name> to narrow it, which
is what the local edit was reaching for.

ansible/vars/greatbear.yml

@@ -11,3 +11,7 @@ docker_network: '{{ docker_container }}_net'
 # Everything the app writes — data/ and the untracked site.env. Deliberately the
 # directory the old static menus were served from.
 app_root: '/var/lib/{{ host }}'
+
+# The host's default Docker pools are exhausted, so every Site names a subnet.
+# This one already exists from the pool; the value is what a rebuild would use.
+network_subnet: 10.210.2.0/24