Fix API and Contract Bugs

Procedure

1. Validate input at the boundary (type, emptiness, length, format) with a schema; bad input returns 400/422 with a plain message, never 500.
2. One allow-list serializer per resource for every response, including create, update, and duplicate paths. Never send raw records.
3. Idempotency: dedupe by key or event_id before side effects; set state, don't increment it.
4. Pagination and limits: clamp limit, reject negative offsets, stable ordering.
5. Serialization: parse dates at boundaries, use integer cents, and send null vs. missing deliberately.
6. Contract diff: compare response shapes before and after; announce every change; never drop a field without checking callers.
7. Re-run schema fuzzing (Schemathesis) and the tester's contract tests.

Exit criteria

Fuzzing returns no 5xx and no schema violations, and the contract diff is empty or explicitly approved.

Counterpart of tester skill: qa-api-contract.