sender: drain backlog with ack-gated catch-up sends

src/main.cpp

@@ -1417,6 +1417,12 @@ static void sender_loop() {
         }
       }
     }
+
+    // Catch-up mode: when backlog exists, send next queued batch without waiting
+    // for the regular 30s cadence.
+    if (!g_batch_ack_pending && g_batch_count > 1) {
+      send_meter_batch(last_sample_ts());
+    }
   } else {
     if (!g_batch_ack_pending && now_ms - g_last_sync_request_ms >= SYNC_REQUEST_INTERVAL_MS) {
       g_last_sync_request_ms = now_ms;