Customer AP bill list (accountant mirror, read-only)

Accountant-scoped read-only mirror of GET /companies/{companyId}/ap (bill read models; the open-bills totals the customer app derives are computed from the same rows). Same conversion gate (404 while dark). Optional status and vendorId filters as on the customer mount. Entity scope is resolved from the customer's own entity roster (accountants hold no per-user entity grants): an explicit entityId must be one of the customer's concrete (non-group) entities; absent, the customer's single concrete entity is used; a multi-entity customer without entityId gets 422 ENTITY_SELECTION_REQUIRED and a customer with no entity gets 422 ENTITY_SCOPE_UNAVAILABLE.