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.

test/site-configs.test.ts

@@ -52,8 +52,6 @@ describe('every Site in the inventory is complete', () => {
       assert.match(varsRaw, /^title: /m, `${name}: ${varFile} has no title`);
       assert.match(varsRaw, /^host: 'menu\.\{\{ title \}\}'/m, `${name}: host is not quoted`);
       assert.match(varsRaw, /^app_root: /m, `${name}: no app_root`);
-      // Without this the play fails on a host whose Docker pools are full.
-      assert.match(varsRaw, /^network_subnet: /m, `${name}: no network_subnet`);
     }
   });