feat: add cursor pagination to audit export

[seonghobae]

Summary

• add paired after_occurred_at and after_audit_event_record_id keyset pagination to tenant audit export
• reject partial cursors with HTTP 400 while preserving the existing array response and bounded limit
• add ADR-0004, PRD/TRD/architecture/runbook traceability, and exact-head PostgreSQL/API smoke coverage

Real PostgreSQL/API evidence

• PostgreSQL 18.4 disposable database and running Rust API
• first page returned 2 events; cursor page returned the remaining 3 in stable order
• partial cursor returned HTTP 400
• second tenant returned an empty array
• source payloads remained excluded

Verification

• cargo +1.97.1 fmt --all -- --check
• cargo +1.97.1 test --workspace --all-targets --locked
• cargo +1.97.1 clippy --workspace --all-targets --locked -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo +1.97.1 doc --workspace --no-deps --locked
• actionlint, docs contract, and git diff --check

Gate

This PR is stacked on PR #18 exact head f0a5e55. It remains subject to independent current-head review, required Checks, and protected merge rules. No self-approval, merge, or protection bypass is requested.

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[seonghobae]

@opencode-agent Review only the exact current head a718bfe against base f0a5e55 for PR #19. Do not update branches, approve, merge, or bypass protection. Check keyset cursor semantics, tenant/RLS isolation, SQL parameter ordering, URL/query validation, backward compatibility, and the real PostgreSQL/API smoke contract.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.