feat: livrable ticket #91 — notification fin BackgroundTask enrichie

- Backend: enrichissement HeadlessRendezvous avec requester/target/conversationId
- Frontend: toast 'Requester -> Target completed/...' explicite
- Clic ouvrant viewer de conversation pour visualiser l'échange

crates/web-server/src/lib.rs

@@ -5874,6 +5874,7 @@ mod tests {
                 project_id,
                 task_id,
                 owner_agent_id: owner,
+                rendezvous: None,
             });
         let frame = tokio::time::timeout(Duration::from_secs(1), rx.recv())
             .await