Fix role enum migration constraint handling

[chrystianmartins]

Motivation

• The 0003_update_user_role_enum_for_new_auth_flow Alembic migration was failing on some Postgres databases because the check constraint backing the non-native Enum(..., native_enum=False) column did not have the expected name, causing psycopg.errors.UndefinedObject during DROP CONSTRAINT.

Description

• Convert the role column to VARCHAR(32) explicitly before remapping values in both upgrade() and downgrade() using ALTER TABLE users ALTER COLUMN role TYPE VARCHAR(32) USING role::text.
• Perform defensive DROP CONSTRAINT IF EXISTS calls for common generated names (role_enum, ck_users_role_enum, users_role_check) and then add an explicit CHECK constraint for the target values before converting back to the Enum type.
• Adjust batch_op.alter_column calls to use existing_type=sa.String(length=32) while converting to/from the sa.Enum types to match the intermediate string column state.
• Changes are in pool-coordinator/alembic/versions/0003_update_user_role_enum_for_new_auth_flow.py and affect both upgrade() and downgrade() flows.

Testing

• Compiled the updated migration file with python -m compileall pool-coordinator/alembic/versions/0003_update_user_role_enum_for_new_auth_flow.py and the compilation succeeded.

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