Announce an acknowledged node failure on the bus

Clearing a node's last error on the engine published nothing, so a second
browser kept the marker until its next snapshot. One event carries the
qualified node; the receiving client drops the marker without refetching.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013Gf7WaExcJ9bs3kfJXB3nK

backend/fluksio/api/routes/flows.py

@@ -779,6 +779,15 @@ def acknowledge_node_error(
         raise HTTPException(
             status_code=404, detail=f"No node named '{node_id}' in flow '{name}'"
         ) from None
+    # Another browser holds the same marker and has no reason to refetch, so it
+    # would keep showing a failure that is gone until its next snapshot.
+    event_bus.publish(
+        {
+            "type": "node_error_acknowledged",
+            "node": f"{name}.{node_id}",
+            "ts": time.time(),
+        }
+    )
     return Message(message=f"Cleared the failure on '{node_id}'")