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

internal/gateway/server.go

@@ -469,11 +469,16 @@ func (g *Gateway) handleStatusAPI(w http.ResponseWriter, r *http.Request) {
 	if reqRole(r.Context()) == "admin" {
 		sts := g.core.Registry().Status()
 		recent := g.stats.SourceRecent(300)
+		avgs := g.stats.SourceAverages(300)
 		for i := range sts {
 			if v, ok := recent[sts[i].Name]; ok {
 				sts[i].RecentOK = v[0]
 				sts[i].RecentErr = v[1]
 			}
+			if a, ok := avgs[sts[i].Name]; ok {
+				sts[i].AvgFirstByteMs = a.AvgFirstByteMs
+				sts[i].AvgTokPerS = a.AvgTokPerS
+			}
 		}
 		resp["sources"] = sts
 		resp["adapters"] = g.core.ListAdapters()