Phoenix Auth Customization

Use this skill when extending phx.gen.auth with custom fields.

RULES — Follow these with no exceptions

1. Never modify generated auth migrations — create separate migrations for custom fields
2. Update registration_changeset to cast and validate new fields — don't create a separate changeset
3. Update test fixtures when adding required fields — missing fixture fields cause cryptic test failures
4. Confirm users in test fixtures for password-based auth — set confirmed_at or tests will fail
5. Update both the registration form AND the save/2 handler — the form must send the field
6. Use unique_constraint + database unique index for uniqueness — never validate in application code alone

Running phx.gen.auth