Both held arbitrary-length data (a raw signed transaction hex, an audit
payload) in a bare String, which SQLAlchemy compiles to VARCHAR with no
length. SQLite and PostgreSQL accept that; other backends like MySQL
require a length on VARCHAR and would reject it. Add a migration
(verified upgrade/downgrade/upgrade round-trip, and confirmed with
`alembic check` that it leaves no further diff against the models).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
