新功能(#69): 用户级限速覆盖 — 数据层 + 核心逻辑 + 管理接口

- 新增 migration 02153: user_subscribe 表添加 speed_limit、traffic_limit 列（幂等）
- model 层 Subscribe/SubscribeDetails 新增用户级限速字段（*string 类型支持 nil 回退）
- server 用户列表和管理员详情接口支持用户级限速覆盖优先级计算
- 更新管理员订阅更新接口支持写入/清除用户级限速

Co-authored-by: multica-agent <github@multica.ai>

internal/logic/server/getServerUserListLogic.go

@@ -307,14 +307,24 @@ func (l *GetServerUserListLogic) canUseExpiredNodeGroup(userSub *user.Subscribe,
 
 // calculateEffectiveSpeedLimit 计算用户的实际限速值（考虑按量限速规则）
 func (l *GetServerUserListLogic) calculateEffectiveSpeedLimit(sub *subscribe.Subscribe, userSub *user.Subscribe) int64 {
+	baseSpeed := sub.SpeedLimit
+	if userSub.SpeedLimit > 0 {
+		baseSpeed = userSub.SpeedLimit
+	}
+
+	trafficLimit := sub.TrafficLimit
+	if userSub.TrafficLimit != nil && *userSub.TrafficLimit != "" {
+		trafficLimit = *userSub.TrafficLimit
+	}
+
 	result := speedlimit.CalculateWithCache(
 		l.ctx.Request.Context(),
 		l.svcCtx.Redis,
 		l.svcCtx.DB,
 		userSub.UserId,
 		userSub.Id,
-		sub.SpeedLimit,
-		sub.TrafficLimit,
+		baseSpeed,
+		trafficLimit,
 		30*time.Second,
 	)
 	return result.EffectiveSpeed