- Add users.password_hash to _migrate_add_missing_columns pending list
- Skip tables that don't exist yet (prevents crash on fresh DB)
- Handle nullable columns (no DEFAULT clause) in ALTER TABLE
- Fix SQLite column type (use sql_type instead of hardcoded TEXT)
- Update migrate_roles.py to create schema columns/tables before seeding roles
- Add unit tests for migration: adds column, nullable, idempotent, no-op
