[Icebear-16]

Summary

• Adds the shared offset/page + limit pagination contract to the remaining high-volume list endpoint, GET /admin/dead-letter.
• Returns pagination metadata in the response envelope and X-Total-Count, with default 50 / max 200 page sizes.
• Adds an occurredAt sort fallback for the domain-event ledger and covers slicing/boundary behavior with integration tests.

Validation

• npm run typecheck
• npx eslint on changed files
• npx vitest run — 1904 tests passed
• npm run docs:check

Closes #297
closes #287
closes #286
closes #295