Lay the groundwork for durable execution

[triggeredcode]

Scope

• Repair migration history to match the current Prisma schema while preserving legacy challenge and submission rows.
• Add a disposable-Postgres migration parity job to CI.
• Define and test versioned execution input and result-provenance contracts.
• Document the ordered rollout of the durable worker and sandbox.

Verified locally

• Empty and populated legacy migration paths, plus an already-db-pushed schema, all reached schema parity in disposable Postgres.
• 99 unit tests, typecheck, lint, repository hygiene, and webpack production build passed.

This PR does not move execution out of the web process or make public untrusted execution safe. The queue, worker, verified bundles, and sandbox are follow-up work.