feat: add per-source TTFB and tokens/s metrics to status page

- Req: add FirstByteMs field (ms to first byte, tracked for streaming)
- Stat: add FirstByteSum for aggregation
- SourceAverages(): new method computing per-source avg TTFB and tokens/s
  from the in-memory ring (300s window)
- SourceStatus: add AvgFirstByteMs and AvgTokPerS fields
- pumpStream: record FirstByteMs after first SSE chunk sent to client
- singleChat/singleChatAuto: set FirstByteMs = LatMs (non-streaming)
- handleStatusAPI: populate the new SourceStatus fields from SourceAverages()
- WebUI source table: two new columns showing TTFB (s) and Tokens/s

.omo/run-continuation/ses_fcda7f5e1ffe9g78A7lVoLMxn7.json

@@ -0,0 +1,10 @@
+{
+  "sessionID": "ses_fcda7f5e1ffe9g78A7lVoLMxn7",
+  "updatedAt": "2026-08-24T11:19:43.949Z",
+  "sources": {
+    "background-task": {
+      "state": "idle",
+      "updatedAt": "2026-08-24T11:19:43.949Z"
+    }
+  }
+}