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/model/user/user.go

@@ -101,10 +101,10 @@ type Subscribe struct {
 	Traffic         int64      `gorm:"default:0;comment:Traffic"`
 	Download        int64      `gorm:"default:0;comment:Download Traffic"`
 	Upload          int64      `gorm:"default:0;comment:Upload Traffic"`
-	SpeedLimit      int64      `gorm:"default:0;comment:User-level speed limit override (Mbps, 0=use plan default)"`
-	TrafficLimit    string     `gorm:"type:text;default:null;comment:User-level traffic limit rules override (JSON)"`
 	ExpiredDownload int64      `gorm:"default:0;comment:Expired period download traffic (bytes)"`
 	ExpiredUpload   int64      `gorm:"default:0;comment:Expired period upload traffic (bytes)"`
+	SpeedLimit      int64      `gorm:"default:0;comment:User-level speed limit override (Mbps), 0 uses plan-level"`
+	TrafficLimit    *string    `gorm:"type:text;default:null;comment:User-level traffic limit override (JSON), NULL uses plan-level"`
 	Token           string     `gorm:"index:idx_token;unique;type:varchar(255);default:'';comment:Token"`
 	UUID            string     `gorm:"type:varchar(255);unique;index:idx_uuid;default:'';comment:UUID"`
 	Status          uint8      `gorm:"type:tinyint(1);default:0;comment:Subscription Status: 0: Pending 1: Active 2: Finished 3: Expired 4: Deducted 5: stopped"`