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.

test/site-configs.test.ts

@@ -52,6 +52,8 @@ 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`);
     }
   });