feat(dashboard): windowed stats per range + clear-logs button

- Stats snapshot now computes the KPI totals AND the breakdown from the
  in-window per-minute buckets (previously they used all-time accumulators, so
  switching 1h/24h/7d changed only the chart, not the numbers). Buckets now
  carry failovers + per-group breakdown; Prometheus keeps all-time counters.
- New DELETE /admin/api/stats clears all in-memory stats and truncates the
  JSONL log; the dashboard gets a "Clear logs" button (with confirm).

src/admin/mod.rs

@@ -165,7 +165,7 @@ pub fn api_routes(admin_token: String) -> Router<SharedState> {
             "/admin/api/secrets",
             get(list_secrets).put(set_secret).delete(clear_secret),
         )
-        .route("/admin/api/stats", get(get_stats))
+        .route("/admin/api/stats", get(get_stats).delete(clear_stats))
         .route("/admin/api/shadow", get(get_shadow))
         .route("/admin/api/hf/search", get(hf_search))
         .route("/admin/api/import", get(list_imports).post(start_import))
@@ -1197,6 +1197,13 @@ async fn get_stats(State(state): State<SharedState>, Query(q): Query<StatsQuery>
     ok(snap)
 }
 
+/// Clear all recorded stats/logs (in-memory aggregates, series, recent requests)
+/// and truncate the JSONL log — the dashboard "Clear logs" button.
+async fn clear_stats(State(state): State<SharedState>) -> ApiResult {
+    state.stats.clear();
+    ok(json!({ "ok": true }))
+}
+
 /// Self-warming shadow loop: per-task-type promotion state, Wilson-LB of the
 /// judged pass-rate, and whether the local model is serving it yet.
 async fn get_shadow(State(state): State<SharedState>) -> ApiResult {