feat: add invitation and promotion functions

api/store/model/invite_code.go

@@ -0,0 +1,12 @@
+package model
+
+import "time"
+
+type InviteCode struct {
+	Id        uint `gorm:"primarykey;column:id"`
+	UserId    uint
+	Code      string
+	Hits      int // 点击次数
+	RegNum    int // 注册人数
+	CreatedAt time.Time
+}