gateway: dial swarm services by name over verified TLS

Consumers reached authelia at `127.0.0.1:<port>`, which encoded a
co-location nobody agreed to: the gateway and authelia are not required
to share a host, so the literal is a requirement stated only by being
unwriteable any other way. Moving them to the name is the point of the
issue.

But a name over https is only half of "https and auth". nginx's
`proxy_ssl_verify` is OFF by default and there was no `proxy_ssl_*`
anywhere in the tree, so the obvious repoint would have produced an
encrypted, unauthenticated hop -- which works, and keeps working,
against any certificate at all.

Adds `gateway.lib.verifiedProxyTo <name>` next to the rest of the vhost
kit, so the convention has one definition rather than a copy in each
consuming module, and repoints the four call sites through it.

Each directive was checked against a real nginx with the opposite arm
run as a control:

  - the CA *bundle* (root + intermediate) is accepted -- worth checking,
    since `hive-ca-trust.nix` warns off consumers that read only one
    certificate, and nginx is not one of those
  - verification checks the chain: an unrelated CA fails
  - and the HOSTNAME: a wrong `proxy_ssl_name` fails even with a good
    chain. Chain-only would accept any cert this CA ever signed, which
    for an internal CA is every service on the hive
  - with verify off, the wrong CA passes -- so the failures above come
    from verification, not from the connection

Bind addresses are untouched. This changes what consumers dial, not what
anything listens on.

nix/host-modules/hive-gateway/default.nix

@@ -90,6 +90,11 @@ let
       swarmServiceDomains
       errorPages
       ;
+    # Same path `lib/hive-ca-trust.nix` publishes as `caHostPath`. Named
+    # from the tls option here rather than importing that helper: this
+    # is the HOST's own bundle, and the helper's job is assembling a
+    # per-container copy.
+    caBundle = "${config.services.hyperhive.tls.stateDir}/trust-bundle.pem";
   };
 
   nginxTree = import ./vhosts.nix {