fix(Migrator): disable foreign keys during DropColumn

[lihan3238]

Closes #119

Problem

DropColumn fails when foreign keys are enabled because it calls recreateTable, which performs DROP TABLE + INSERT INTO + ALTER TABLE RENAME. SQLite enforces FK constraints on the INSERT INTO step, causing the operation to fail with constraint violation errors.

Solution

Wrap DropColumn in RunWithoutForeignKey, following the exact same pattern already used by AlterColumn and DropTable in this file.

Before

func (m Migrator) DropColumn(value interface{}, name string) error {
    return m.recreateTable(value, nil, func(...) { ... })
}

After

func (m Migrator) DropColumn(value interface{}, name string) error {
    return m.RunWithoutForeignKey(func() error {
        return m.recreateTable(value, nil, func(...) { ... })
    })
}