feat(c0re): /api/stats-hive — hive-wide turn-stats rollup

#1424 P2 backend. New hive_stats module aggregates every agent's
turn-stats.sqlite read-only (reusing Coordinator::kept_state_names +
agent_harness_dir, skipping missing/unreadable dbs) into swarm totals,
a busiest-first per-agent rollup, swarm model mix, and a labelled USD
cost estimate (rough model->price table; can move to a nix option
later). Exposed as GET /api/stats-hive?window=. Dashboard UI follows.

hive-c0re/src/dashboard.rs

@@ -67,6 +67,7 @@ pub async fn serve(port: u16, coord: Arc<Coordinator>) -> Result<()> {
         .route("/api/approval-diff/{id}", get(get_approval_diff))
         .route("/api/state-file", get(get_state_file))
         .route("/api/reminders", get(api_reminders))
+        .route("/api/stats-hive", get(api_stats_hive))
         .route("/api/build-logs", get(get_build_logs_all))
         .route("/api/build-logs/{agent}", get(get_build_logs_agent))
         .route("/api/build-logs/id/{id}", get(get_build_log_full))
@@ -1734,6 +1735,19 @@ async fn api_reminders(State(state): State<AppState>) -> Response {
     }
 }
 
+#[derive(Deserialize)]
+struct StatsHiveQuery {
+    window: Option<String>,
+}
+
+/// Hive-wide turn-stats rollup for the dashboard swarm-stats view.
+/// Aggregates every agent's `hyperhive-turn-stats.sqlite` read-only
+/// (skips missing/unreadable ones). Window defaults to `24h`.
+async fn api_stats_hive(axum::extract::Query(q): axum::extract::Query<StatsHiveQuery>) -> Response {
+    let window = crate::hive_stats::Window::parse(q.window.as_deref().unwrap_or("24h"));
+    axum::Json(crate::hive_stats::hive_snapshot(window)).into_response()
+}
+
 #[derive(Deserialize)]
 struct BuildLogsQuery {
     /// Maximum number of rows to return. Capped server-side at 50