Backend: Runtime-Safe Migrations

Schema/data migrations must be runtime-safe.

Scope:
- Enforce when PRs include backend migrations or migration-dependent runtime code.

Rules:
1) Migration-related runtime changes should preserve compatibility during rollout (old and new code/data overlap window).
2) Avoid coupling destructive schema changes with first-use runtime logic in the same unsafe step unless clearly justified.
3) PR description should include brief rollout/compatibility notes when runtime behavior depends on migration state.

Review behavior:
- Return NOT APPROVED only when migration/runtime coupling appears unsafe or rollout compatibility is unclear.
- Do not block purely additive migrations with clear compatibility.