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/roles/network/tasks/main.yaml

@@ -1,5 +1,29 @@
 ---
-- name: 'Ensure {{ docker_network }} exists and Caddy is on it'
+- name: Look for the app network
+  docker_network_info:
+    name: '{{ docker_network }}'
+  register: menu_network
+
+- name: Fail when this Site declares no subnet
+  fail:
+    msg: >-
+      {{ inventory_hostname }} needs a network_subnet in its vars file. This host's
+      default address pools are exhausted (every 172.17-172.30/16 and every
+      192.168.0-240/20 is taken), so Docker refuses to allocate one and a network
+      has to be created with an explicit subnet.
+  when: network_subnet | default('') == ''
+
+# An existing network is left exactly as it is: Docker cannot change a network's
+# subnet in place, and the Sites already running have theirs.
+- name: Create the app network with this Site's subnet
+  docker_network:
+    name: '{{ docker_network }}'
+    ipam_config:
+      - subnet: '{{ network_subnet }}'
+    state: present
+  when: not menu_network.exists
+
+- name: Put Caddy on the app network
   docker_network:
     name: '{{ docker_network }}'
     state: present