feat: live bandwidth tracking (RX/TX MB/s + avg response size)

- Track data_received / data_sent bytes in live metric aggregation
- Compute bwInMBps, bwOutMBps, avgRespKB per second
- Bandwidth row below stats strip: ↓ RX | ↑ TX | Avg size
- RX colour: green <20 MB/s, yellow 20-50 MB/s, red >50 MB/s
- Warning banner when RX >20 MB/s: bandwidth may cause p99 spikes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/server.js

@@ -171,12 +171,13 @@ app.post('/api/tests', (req, res) => {
 
   // Live metric aggregation state
   const live = {
-    durations: [],       // http_req_duration values (ms)
-    p90durations: [],    // sampled for percentile calc
+    durations: [],
     totalReqs: 0,
     failedReqs: 0,
     checksTotal: 0,
     checksPassed: 0,
+    bytesIn: 0,          // data_received (bytes)
+    bytesOut: 0,         // data_sent (bytes)
     startTime: Date.now(),
     lastBroadcast: 0,
   };
@@ -317,6 +318,10 @@ function ingestPoint(obj, live) {
   } else if (metric === 'checks') {
     live.checksTotal++;
     if (data.value === 1) live.checksPassed++;
+  } else if (metric === 'data_received') {
+    live.bytesIn += data.value;
+  } else if (metric === 'data_sent') {
+    live.bytesOut += data.value;
   }
 }
 
@@ -334,6 +339,12 @@ function computeLiveMetrics(live) {
   const p95 = pct(95);
   const score = computeScore({ p95, httpErrRate, checksRate });
 
+  const bwInMBps  = +(live.bytesIn  / elapsed / 1048576).toFixed(2); // MB/s
+  const bwOutMBps = +(live.bytesOut / elapsed / 1048576).toFixed(2);
+  const avgRespKB = live.totalReqs > 0
+    ? +(live.bytesIn / live.totalReqs / 1024).toFixed(1)
+    : 0;
+
   return {
     totalReqs:   live.totalReqs,
     reqPerSec:   +(live.totalReqs / elapsed).toFixed(1),
@@ -349,6 +360,9 @@ function computeLiveMetrics(live) {
     p99:         +pct(99).toFixed(1),
     max:         sorted.length ? +sorted[sorted.length - 1].toFixed(1) : 0,
     score,
+    bwInMBps,
+    bwOutMBps,
+    avgRespKB,
   };
 }