Alembic batch operations for SQLite compatibility

Need to run Alembic migrations that work on both PostgreSQL and SQLite. SQLite doesn't support ALTER TABLE for column renames or type changes, causing migration failures in test environments.

Use Alembic batch operations context manager:

with op.batch_alter_table('users') as batch_op:
    batch_op.alter_column('name', new_column_name='display_name')
    batch_op.alter_column('age', type_=sa.String(10))

Batch mode recreates the table with changes applied atomically. Set render_as_batch=True in env.py for automatic batch wrapping.