add postgres and refactor code

model/organization.go

@@ -8,3 +8,8 @@ type Organization struct {
 	ParentID  uint       `json:"parent_id"`
 	Positions []Position `json:"positions,omitempty"`
 }
+
+type OrganizationApi struct {
+	Name     string `json:"name" gorm:"unique;index"`
+	ParentID uint   `json:"parent_id"`
+}