feat(stovepipe): persist request log records - #648

Summary

Intent:

• Make the request-log schema usable through a backend-neutral, queue-scoped persistence contract.
• Supply the retained records that the public request-history API projects as HistoryEvent values.

Changes:

• Define validated RequestLog records with bounded state, event, and outcome-reason vocabularies plus opaque metadata.
• Add create, point-read, and complete per-request list operations with no pagination.
• Round-trip metadata through MySQL JSON without interpreting its keys or values, writing nil as {} and normalizing stored JSON null to an empty map.
• Keep the raw metadata map out of the public protobuf; key-specific enforcement is deferred.
• Implement and wire the MySQL store, generated mocks, and reusable backend contract tests.

Test Plan

• Exercised metadata round trips, nil normalization, invalid JSON value handling, duplicate detection, point reads, queue isolation, and chronological listing.
• ./tool/bazel test //api/stovepipe/... //service/stovepipe/... //stovepipe/...
• make lint check-tidy check-gazelle

Revert Plan

Revert this PR; the table introduced by #645 remains unused.

Jira Issues

None.