Add `deleted` terminal status: completed/cancelled/failed → deleted via
DELETE /api/intelligent-evals/{id} (idempotent, 409 for non-terminal).
Deleted evals are hidden from list, detail, stats, and all sub-resource
endpoints (sessions/report/decision-logs/config-snapshots); child tables
are untouched (audit-safe). Frontend shows a Popconfirm-guarded delete
button for terminal evals only.