feat: add delete schoolyear route and handler

backend/database.go

@@ -580,3 +580,31 @@ func calculateHours(entry TimeEntry) float64 {
 		return calculateHoursDiff(entry.StartTime, entry.EndTime)
 	}
 }
+
+func DeleteSchoolYear(db *sql.DB, id int) error {
+	var isActive bool
+	err := db.QueryRow("SELECT is_active FROM school_years WHERE id = ?", id).Scan(&isActive)
+	if err != nil {
+		return err
+	}
+
+	if isActive {
+		return fmt.Errorf("cannot delete active school year")
+	}
+
+	result, err := db.Exec("DELETE FROM school_years WHERE id = ? AND is_active = 0", id)
+	if err != nil {
+		return err
+	}
+
+	rowsAffected, err := result.RowsAffected()
+	if err != nil {
+		return err
+	}
+
+	if rowsAffected == 0 {
+		return sql.ErrNoRows
+	}
+
+	return nil
+}