migrations: remove .backup development artifacts before 2.0.0 tag

11 .backup files in src/migrations/ were mid-refactor snapshots left
behind during v2 schema development. They duplicate active migrations
and have no runtime purpose (Laravel's migration loader only picks up
`.php` files, so they were inert). Clean them up so the tagged release
ships only the 24 canonical create-table migrations.

Closes #292.

Author: durable-workflow-ops