robustness: shared HTTP client, HTTP status checks, abort controller, streaming perf

- web.go: reuse shared binanceClient with connection pooling instead of per-request client
- sentinel.go: check HTTP status code before parsing Binance ticker response
- brain.go: check HTTP status codes in news scan and market brief APIs
- brain.go: add cleanStaleSignals() to prevent unbounded sync.Map growth
- scheduler.go: periodically clean expired pending trades
- AgentChatPage.tsx: add AbortController to cancel in-flight requests
- AgentChatPage.tsx: check response.ok before parsing JSON
- AgentChatPage.tsx: batch word streaming (~40 frames max) to reduce re-renders
- AgentChatPage.tsx: handle AbortError gracefully (remove orphan bot message)

agent/sentinel.go

@@ -116,6 +116,10 @@ func (s *Sentinel) check(symbol string) {
 	resp, err := s.http.Get(fmt.Sprintf("https://fapi.binance.com/fapi/v1/ticker/24hr?symbol=%s", symbol))
 	if err != nil { return }
 	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		s.logger.Debug("sentinel ticker non-200", "symbol", symbol, "status", resp.StatusCode)
+		return
+	}
 	body, err := io.ReadAll(io.LimitReader(resp.Body, 256*1024)) // 256KB limit
 	if err != nil { return }
 	var t map[string]interface{}