swarm-controller: add configured default matrix homeserver URL

Adds services.hyperhive.deploy.swarm-controller.matrixHomeserverUrl,
threaded to the daemon as SWARM_CONTROLLER_MATRIX_HOMESERVER_URL, and a
Rust helper (homeserver_or_configured_default) that lets a caller-supplied
homeserver keep overriding it. Config plumbing only: put_matrix_account
does not call the helper yet, so this is a no-op for every current caller.

Refs #4345

swarm-controller/src/main.rs

@@ -1621,6 +1621,17 @@ async fn main() -> Result<()> {
         }
     };
 
+    // Diagnostic only — `put_matrix_account` does not call this yet (see its
+    // own doc comment), so this cannot change any request's outcome. Logged
+    // so an operator can tell the config landed before the slice that
+    // consults it does.
+    if matrix_account::homeserver_or_configured_default(None).is_none() {
+        tracing::debug!(
+            "{}; matrix-account requests still need their own homeserver",
+            matrix_account::DEFAULT_HOMESERVER_ENV
+        );
+    }
+
     let config_prs = forge_client.clone().map(config_pr::spawn);
     let state_forge = keep_forge_for_state(forge_client, webhook_secret.clone());