swarm-controller: read the queue client secret from the store, drop the file

The controller's OIDC client secret (client `swarm-controller`, used for
the queue connection, the auth-bridge bearer and the OTLP push) came from
an operator-placed file, `deploy.swarm-controller.queue.clientSecretFile`,
handed in by `LoadCredential=`.

Now `swarm-secret-publish`, which already copies authelia's minted OIDC
secrets into the store, also publishes this one, to
`swarm/controller/swarm-controller/oidc/client`. That path sits under
`controller/`, which no hive's policy reads. The controller reads it once
at start with its existing store certificate and holds it in memory, as
`swarm_queue_client::ClientSecret::Value`. If the store is down, it
retries for about a minute and then fails the start, so `Restart=` tries
again.

Policy delta: the controller gets `read` on that leaf, and the publisher
gets `create`/`update` on that leaf.

Removed: the `queue.clientSecretFile` option (both spellings, now removed
options with a message), its singleHostSwarm default, the credential and
placeholder, and the path watcher plus its restart oneshot. A controller
without a store identity is now an eval error, because it has no other
way to get the secret.

nix/module-eval/secret-publisher.nix

@@ -85,6 +85,20 @@ let
     deploy.matrix.enable = true;
   };
   cases = [
+    {
+      # The controller reads this exact path
+      # (`swarm_secret_client::queue::controller_client_path`, pinned by that
+      # crate's own test). Not under `services/`, which every hive reads: this
+      # client may read every hive's status.
+      name = "the publisher writes the controller's client under controller/, never services/";
+      ok =
+        let
+          s = secretPublisherHere.systemd.services.swarm-secret-publish.script;
+        in
+        lib.hasInfix "secret/swarm/controller/swarm-controller/oidc/client" s
+        && lib.hasInfix "/swarm-controller.secret" s
+        && !(lib.hasInfix "secret/swarm/services/swarm-controller" s);
+    }
     {
       # Both ends of a wire nothing at eval time carries end to end: the
       # publisher on authelia's host writes the path the reader on Grafana's host