Add delete goal endpoint (#27)

Closes #18

Reviewed-on: #27

backend/db.go

@@ -128,3 +128,25 @@ func (db *Db) CreateGoal(userId int, goal *CreateGoalRequest) (int, error) {
 
 	return lastId, nil
 }
+
+func (db *Db) DeleteGoal(userId int, goalId int) error {
+	// Check if the goal exists for the user
+	count := 0
+	err := db.db.Query("select count(*) from goals where id = ? and user_id = ?").
+		Bind(goalId, userId).ScanSingle(&count)
+	if err != nil {
+		return err
+	}
+	if count == 0 {
+		return errors.New("goal not found")
+	}
+
+	// Delete the goal
+	err = db.db.Query("delete from goals where id = ? and user_id = ?").
+		Bind(goalId, userId).Exec()
+	if err != nil {
+		return err
+	}
+
+	return nil
+}