swarm-controller: keep retrying the forge webhook secret and registration after a startup outage

The webhook secret slot is filled in the background, on the forge token's
capped unbounded backoff, when the store can't answer at start. The hook
registration waits for the secret and retries on the same backoff instead
of giving up after five tries.

Closes #6101

swarm-controller/src/backend_env.rs

@@ -164,7 +164,7 @@ mod tests {
                 hive_jobq::Graph::new(),
                 hive_jobq::resources::ResourceTable::new(),
             ))),
-            webhook_secret: None,
+            webhook_secret: std::sync::Arc::default(),
             config_prs: std::sync::Arc::default(),
             swarm_name: None,
             auth: None,