Billing Reconciliation

Completed-but-unbilled interviews — the silent billing-failure detector.

Lists chargeable interviews that reached status='completed' but have no deduction row, i.e. billing never fired. A capped action still writes a $0 audit row, so a missing row is a real miss, not a free action. Internal-staff only (404 to everyone else, same as the rest of this router). Read-only — no mutation, no backfill from here.