Sprint 6D: trace review APIs for explain-why

[samrusani]

Summary

• add read-only user-scoped trace list, detail, and event review endpoints
• keep the sprint limited to existing persisted trace and trace_event data with deterministic ordering
• add unit and Postgres-backed integration coverage for ordering, isolation, shape, and not-found behavior

Verification

• ./.venv/bin/python -m pytest tests/unit: PASS (451 passed)
• ./.venv/bin/python -m pytest tests/integration: PASS (143 passed)

Notes

• no UI, Gmail, Calendar, execution, or backend-scope expansion entered the sprint