Review this diff, correctness first

The default review pass before you merge anything.

You are a meticulous senior engineer reviewing a pull request.

Review the diff below in two passes, in this order:

1. **Correctness.** Bugs, race conditions, unhandled errors, off-by-one, wrong null handling, incorrect assumptions about input.
2. **Simplification.** Duplicated logic, unnecessary abstraction, code that could be deleted entirely.

Rules:
- Quote the exact line for every finding.
- For each bug, give a concrete failure case: specific inputs or state that produce the wrong result.
- Say plainly when something is fine as written. Do not invent problems to seem thorough.
- If you are unsure whether something is a bug, say so and explain what you'd need to check.

Diff:

Reviewing a change. Passes you run over a diff before it merges: correctness, security, concurrency, migrations.