fix(admin-api): return 202 Accepted with miroir_task_id for topology ops

Update add_node and drain_node endpoints to return 202 Accepted with
miroir_task_id in the response, matching the P4.6 spec.

Changes:
- add_node now returns 202 with miroir_task_id (rebalance:default)
- drain_node now returns 202 with miroir_task_id (rebalance:default)
- Both endpoints include task ID in logging for observability
- Added response shape documentation to both endpoints

Closes: miroir-mkk.6

crates/miroir-proxy/src/routes/admin_endpoints.rs

@@ -1466,6 +1466,16 @@ where
 /// }
 /// ```
 ///
+/// Response (202 Accepted):
+/// ```json
+/// {
+///   "miroir_task_id": "rebalance:default",
+///   "node_id": "node-new",
+///   "replica_group": 0,
+///   "status": "accepted"
+/// }
+/// ```
+///
 /// Implements plan §2 "Adding a node to an existing group":
 /// 1. Add node to topology in `Joining` state
 /// 2. Send `NodeAdded` event to rebalancer worker
@@ -1473,7 +1483,7 @@ where
 pub async fn add_node<S>(
     State(state): State<S>,
     Json(body): Json<serde_json::Value>,
-) -> Result<Json<serde_json::Value>, (StatusCode, String)>
+) -> Result<(StatusCode, Json<serde_json::Value>), (StatusCode, String)>
 where
     S: Clone + Send + Sync + 'static,
     AppState: FromRef<S>,
@@ -1526,11 +1536,12 @@ where
     }
 
     // Send event to rebalancer worker (if available)
+    let index_uid = "default";
     if let Some(ref worker) = app_state.rebalancer_worker {
         let event = TopologyChangeEvent::NodeAdded {
             node_id: id.clone(),
             replica_group,
-            index_uid: "default".to_string(),
+            index_uid: index_uid.to_string(),
         };
         if let Err(e) = worker.event_sender().try_send(event) {
             error!(error = %e, node_id = %id, "failed to send NodeAdded event to rebalancer worker");
@@ -1541,12 +1552,22 @@ where
         }
     }
 
-    info!(node_id = %id, replica_group, "Node addition queued for rebalancing");
-    Ok(Json(serde_json::json!({
-        "node_id": id,
-        "replica_group": replica_group,
-        "message": format!("Node {} added to replica group {}, rebalancing will start shortly", id, replica_group),
-    })))
+    let job_id = miroir_core::rebalancer_worker::RebalanceJobId::new(index_uid);
+    info!(
+        node_id = %id,
+        replica_group,
+        miroir_task_id = %job_id.0,
+        "Node addition queued for rebalancing"
+    );
+    Ok((
+        StatusCode::ACCEPTED,
+        Json(serde_json::json!({
+            "miroir_task_id": job_id.0,
+            "node_id": id,
+            "replica_group": replica_group,
+            "status": "accepted",
+        })),
+    ))
 }
 
 /// DELETE /_miroir/nodes/{id} — Remove a node from the cluster.
@@ -1629,6 +1650,16 @@ where
 
 /// POST /_miroir/nodes/{id}/drain — Drain a node (prepare for removal).
 ///
+/// Response (202 Accepted):
+/// ```json
+/// {
+///   "miroir_task_id": "rebalance:default",
+///   "node_id": "node-0",
+///   "replica_group": 0,
+///   "status": "draining"
+/// }
+/// ```
+///
 /// Implements plan §2 node drain flow:
 /// 1. Mark node as `draining`
 /// 2. Send `NodeDraining` event to rebalancer worker
@@ -1636,7 +1667,7 @@ where
 pub async fn drain_node<S>(
     State(state): State<S>,
     Path(node_id): Path<String>,
-) -> Result<Json<serde_json::Value>, (StatusCode, String)>
+) -> Result<(StatusCode, Json<serde_json::Value>), (StatusCode, String)>
 where
     S: Clone + Send + Sync + 'static,
     AppState: FromRef<S>,
@@ -1688,10 +1719,11 @@ where
     };
 
     // Send event to rebalancer worker
+    let index_uid = "default";
     let event = TopologyChangeEvent::NodeDraining {
         node_id: node_id.clone(),
         replica_group,
-        index_uid: "default".to_string(),
+        index_uid: index_uid.to_string(),
     };
 
     if let Err(e) = worker.event_sender().try_send(event) {
@@ -1702,12 +1734,22 @@ where
         ));
     }
 
-    info!(node_id = %node_id, replica_group, "Node drain queued for rebalancing");
-    Ok(Json(serde_json::json!({
-        "node_id": node_id,
-        "replica_group": replica_group,
-        "message": format!("Node {} is draining, migrations will start shortly", node_id),
-    })))
+    let job_id = miroir_core::rebalancer_worker::RebalanceJobId::new(index_uid);
+    info!(
+        node_id = %node_id,
+        replica_group,
+        miroir_task_id = %job_id.0,
+        "Node drain queued for rebalancing"
+    );
+    Ok((
+        StatusCode::ACCEPTED,
+        Json(serde_json::json!({
+            "miroir_task_id": job_id.0,
+            "node_id": node_id,
+            "replica_group": replica_group,
+            "status": "draining",
+        })),
+    ))
 }
 
 /// POST /_miroir/nodes/{id}/fail — Mark a node as failed.