Let Docker allocate the network subnet again

Reverts the explicit subnets: the pools were freed by deleting five networks, so
the network role is back to its single idempotent task - ensure the network
exists and Caddy is on it - and the Sites no longer declare a subnet. The
deployment readme keeps the note that a new Site needs a config, and therefore a
new tag, but no longer asks for a free /24.

If the pools fill up again the failure is recognisable: 'all predefined address
pools have been fully subnetted', and the choices are deleting stale networks or
declaring a subnet per Site.

ansible/roles/network/tasks/main.yaml

@@ -1,29 +1,5 @@
 ---
-- 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
+- name: 'Ensure {{ docker_network }} exists and Caddy is on it'
   docker_network:
     name: '{{ docker_network }}'
     state: present