feat: 限速状态可视化 - 后台订阅详情展示实际限速与降速状态

- 新增 pkg/speedlimit/calculator.go 公共限速计算函数
- UserSubscribeDetail 响应新增 effective_speed/is_throttled/throttle_rule 字段
- GetUserSubscribeById 接口查询时实时计算并返回限速状态
- 重构 getServerUserListLogic 的 calculateEffectiveSpeedLimit，改用共享函数

Co-Authored-By: claude-flow <ruv@ruv.net>

pkg/speedlimit/calculator.go

@@ -0,0 +1,105 @@
+package speedlimit
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// TrafficLimitRule represents a dynamic speed throttling rule.
+type TrafficLimitRule struct {
+	StatType     string `json:"stat_type"`
+	StatValue    int64  `json:"stat_value"`
+	TrafficUsage int64  `json:"traffic_usage"`
+	SpeedLimit   int64  `json:"speed_limit"`
+}
+
+// ThrottleResult contains the computed speed limit status for a user subscription.
+type ThrottleResult struct {
+	BaseSpeed     int64   `json:"base_speed"`      // Plan base speed limit (Mbps, 0=unlimited)
+	EffectiveSpeed int64  `json:"effective_speed"`  // Current effective speed limit (Mbps)
+	IsThrottled   bool    `json:"is_throttled"`     // Whether the user is currently throttled
+	ThrottleRule  string  `json:"throttle_rule"`    // Description of the matched rule (empty if not throttled)
+	UsedTrafficGB float64 `json:"used_traffic_gb"`  // Traffic used in the matched rule's window (GB)
+}
+
+// Calculate computes the effective speed limit for a user subscription,
+// considering traffic-based throttling rules.
+func Calculate(ctx context.Context, db *gorm.DB, userId, subscribeId, baseSpeedLimit int64, trafficLimitJSON string) *ThrottleResult {
+	result := &ThrottleResult{
+		BaseSpeed:      baseSpeedLimit,
+		EffectiveSpeed: baseSpeedLimit,
+	}
+
+	if trafficLimitJSON == "" {
+		return result
+	}
+
+	var rules []TrafficLimitRule
+	if err := json.Unmarshal([]byte(trafficLimitJSON), &rules); err != nil {
+		return result
+	}
+
+	if len(rules) == 0 {
+		return result
+	}
+
+	now := time.Now()
+	for _, rule := range rules {
+		var startTime time.Time
+
+		switch rule.StatType {
+		case "hour":
+			if rule.StatValue <= 0 {
+				continue
+			}
+			startTime = now.Add(-time.Duration(rule.StatValue) * time.Hour)
+		case "day":
+			if rule.StatValue <= 0 {
+				continue
+			}
+			startTime = now.AddDate(0, 0, -int(rule.StatValue))
+		default:
+			continue
+		}
+
+		var usedTraffic struct {
+			Upload   int64
+			Download int64
+		}
+		err := db.WithContext(ctx).
+			Table("traffic_log").
+			Select("COALESCE(SUM(upload), 0) as upload, COALESCE(SUM(download), 0) as download").
+			Where("user_id = ? AND subscribe_id = ? AND timestamp >= ? AND timestamp < ?",
+				userId, subscribeId, startTime, now).
+			Scan(&usedTraffic).Error
+
+		if err != nil {
+			continue
+		}
+
+		usedGB := float64(usedTraffic.Upload+usedTraffic.Download) / (1024 * 1024 * 1024)
+
+		if usedGB >= float64(rule.TrafficUsage) {
+			if rule.SpeedLimit > 0 {
+				if result.EffectiveSpeed == 0 || rule.SpeedLimit < result.EffectiveSpeed {
+					result.EffectiveSpeed = rule.SpeedLimit
+					result.IsThrottled = true
+					result.UsedTrafficGB = usedGB
+
+					statLabel := "小时"
+					if rule.StatType == "day" {
+						statLabel = "天"
+					}
+					result.ThrottleRule = fmt.Sprintf("%d%s内超%dGB，限速%dMbps",
+						rule.StatValue, statLabel, rule.TrafficUsage, rule.SpeedLimit)
+				}
+			}
+		}
+	}
+
+	return result
+}