Knex was already a dependency but used only as a query builder: no knexfile,
no migration scripts, and no rule stating that schema changes must go through
migrations - unlike the Python template, which has a complete Alembic setup.

- knexfile.ts reading the same DB_* env vars as DatabaseService
- migrate:make / migrate:latest / migrate:rollback / migrate:status scripts
- inert .ts.example migration as a reference (never loaded, built or linted)
- exclude migrations from jest coverage collection
- document the mandate in the base steering and in the README

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>