fix: Setup Token 账号使用真实 utilization 值替代状态估算

从响应头 anthropic-ratelimit-unified-5h-utilization 获取并存储真实
utilization 值，解决进度条始终显示 0% 的问题。窗口重置时清除旧值，
避免残留上个窗口的数据。

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

backend/internal/service/ratelimit_service.go

@@ -970,12 +970,27 @@ func (s *RateLimitService) UpdateSessionWindow(ctx context.Context, account *Acc
 		windowStart = &start
 		windowEnd = &end
 		slog.Info("account_session_window_initialized", "account_id", account.ID, "window_start", start, "window_end", end, "status", status)
+		// 窗口重置时清除旧的 utilization，避免残留上个窗口的数据
+		_ = s.accountRepo.UpdateExtra(ctx, account.ID, map[string]any{
+			"session_window_utilization": nil,
+		})
 	}
 
 	if err := s.accountRepo.UpdateSessionWindow(ctx, account.ID, windowStart, windowEnd, status); err != nil {
 		slog.Warn("session_window_update_failed", "account_id", account.ID, "error", err)
 	}
 
+	// 存储真实的 utilization 值（0-1 小数），供 estimateSetupTokenUsage 使用
+	if utilStr := headers.Get("anthropic-ratelimit-unified-5h-utilization"); utilStr != "" {
+		if util, err := strconv.ParseFloat(utilStr, 64); err == nil {
+			if err := s.accountRepo.UpdateExtra(ctx, account.ID, map[string]any{
+				"session_window_utilization": util,
+			}); err != nil {
+				slog.Warn("session_window_utilization_update_failed", "account_id", account.ID, "error", err)
+			}
+		}
+	}
+
 	// 如果状态为allowed且之前有限流，说明窗口已重置，清除限流状态
 	if status == "allowed" && account.IsRateLimited() {
 		if err := s.ClearRateLimit(ctx, account.ID); err != nil {