ask_operator: operator-side ✗ CANC3L on pending questions

new POST /cancel-question/{id} resolves a pending operator question
with the sentinel answer '[cancelled]' and fires the usual
HelperEvent::OperatorAnswered so the manager sees a terminal state
and can fall back. uses the same OperatorQuestions::answer path —
no special handling, the manager already has to deal with arbitrary
answer strings.

dashboard renders the cancel as a separate <form> below the main
qform so the answer-merge submit handler on the main form doesn't
inadvertently fire when the operator clicks cancel. confirm dialog
spells out what the manager will see.

ttl-based auto-cancel is still on the todo (would spawn a tokio task
per submitted question).

hive-c0re/src/dashboard.rs

@@ -51,6 +51,7 @@ pub async fn serve(port: u16, coord: Arc<Coordinator>) -> Result<()> {
         .route("/rebuild/{name}", post(post_rebuild))
         .route("/update-all", post(post_update_all))
         .route("/answer-question/{id}", post(post_answer_question))
+        .route("/cancel-question/{id}", post(post_cancel_question))
         .route("/purge-tombstone/{name}", post(post_purge_tombstone))
         .route("/request-spawn", post(post_request_spawn))
         .route("/messages/stream", get(messages_stream))
@@ -417,6 +418,33 @@ async fn post_answer_question(
     }
 }
 
+/// Resolve a pending operator question with a sentinel answer when
+/// the operator decides not to / can't answer. The manager harness
+/// receives an `OperatorAnswered` event with `answer = "[cancelled]"`
+/// so it can fall back on whatever default it had. Same code path as
+/// a real answer — just lets the operator close the loop instead of
+/// letting the question dangle forever.
+async fn post_cancel_question(
+    State(state): State<AppState>,
+    AxumPath(id): AxumPath<i64>,
+) -> Response {
+    const SENTINEL: &str = "[cancelled]";
+    match state.coord.questions.answer(id, SENTINEL) {
+        Ok(question) => {
+            tracing::info!(%id, "operator cancelled question");
+            state
+                .coord
+                .notify_manager(&hive_sh4re::HelperEvent::OperatorAnswered {
+                    id,
+                    question,
+                    answer: SENTINEL.to_owned(),
+                });
+            Redirect::to("/").into_response()
+        }
+        Err(e) => error_response(&format!("cancel-question {id} failed: {e:#}")),
+    }
+}
+
 async fn post_purge_tombstone(
     State(state): State<AppState>,
     AxumPath(name): AxumPath<String>,