Audit Prisma transactions for D1 / Cloudflare migration

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Follow-up from docs/agents/d1-prisma-transaction-audit.md.

Checklist:

• Confirm the final Prisma+D1 adapter behavior for array-form $transaction.
• Confirm whether interactive Prisma transactions are supported and safe for the deployed Worker shape.
• Replace migrateHomeworkCompletionsToUser with a D1-safe sequential or batched migration flow.
• Make Call Kent publish cleanup retry-safe after Transistor publish, likely with an upsert keyed by transistorEpisodeId.
• Decide whether Call Kent draft replacement must be atomic on D1 or can rely on admin retry.
• Preserve atomic or retry-safe password update plus session invalidation for both reset-password and change-password routes.
• Add D1/staging tests or manual verification notes for each updated write path before production cutover.