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.

swarm-secret-client/src/matrix.rs

@@ -95,7 +95,7 @@ pub fn appservice_token_path(hive: &str) -> Result<String, Error> {
 
 /// The name segment of the controller's own subtree, and the cert-auth role it
 /// logs in under (`swarm-controller`'s `store::CERT_ROLE`).
-const CONTROLLER: &str = "swarm-controller";
+pub(crate) const CONTROLLER: &str = "swarm-controller";
 
 /// The path holding the **swarm's** appservice token: the one registration
 /// on the swarm's homeserver that is not a hive's, whose sender is promoted