Pytest suite for critical regressions from audit rounds.

Covers:
- `/api/scenario/run` status mapping: `lock_check=None -> 503`, `False -> 409`.
- Unique violation on `scenario_runs` insert maps to `409`.
- `db.scenario_defs.lock_check` DB-unavailable semantics (`None`).
- Static guards for DB partial unique index and `escName` escape chain.

This suite is intentionally lightweight and mock-heavy.
No integration DB or external API calls are required.