nix: ship the journals of the units an apply can leave failed

The units on the path a deploy takes to TLS, the store's grants and the
swarm collector itself were not on the host collector's journald
allowlist, so an ingest outage one of them caused showed in the store
only as every source going quiet at once.

Each module names its own units, per the option's rule:
- hive-tls.nix: hive-tls-ca, swarm-services-cert
- hive-gateway: hive-gateway-self-signed-cert (self-signed mode only)
- swarm-bao.nix: the seven grant units beside
  swarm-bao-services-issuer-policy
- swarm-otel.nix: container@<machine>, and
  nixos-rebuild-switch-to-configuration, the transient unit nixos-rebuild
  runs the activation in and whose syslog lines carry its status

The module-eval arm pins each unit as both listed and defined, since a
listed name that matches nothing is silent.

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

@@ -129,8 +129,13 @@ in
     # Every request to every hyperhive service passes through here, so this
     # is the one unit that can say a service was unreachable rather than
     # merely quiet. Named even on hives that run no swarm collector: the
-    # option is inert unless one is collecting on this host.
-    services.hyperhive.swarm.otel.journaldUnits = [ "nginx" ];
+    # option is inert unless one is collecting on this host. nginx
+    # `Requires=` the self-signed copy, so its journal is the other half of
+    # why nginx did not start.
+    services.hyperhive.swarm.otel.journaldUnits = [
+      "nginx"
+    ]
+    ++ lib.optional useSelfSigned "hive-gateway-self-signed-cert";
 
     assertions = [
       {