Huge refactoring, resolved tight coupling

src/internal/domain/entity/user.go

@@ -0,0 +1,18 @@
+package entity
+
+import "time"
+
+type User struct {
+	ID         int       `json:"id"`
+	Username   string    `json:"username"`
+	Password   string    `json:"password"`
+	CreateDate time.Time `json:"create_date"`
+	ModifyDate time.Time `json:"modify_date"` // FIXME: zero-value issue
+}
+
+var TestUser = &User{
+	ID:         1,
+	Username:   "test",
+	Password:   "test",
+	CreateDate: time.Now(),
+}