Get accounts payable statistics

    Retrieve statistics for all Accounts Payable entities grouped by status.

    Returns counts for:
    - **Bills**: all, unpaid (received + partially_paid), paid
    - **Vendor Credits**: all, unapplied, applied
    - **Vendor Refunds**: all, pending (to get non-pending, use list endpoint with status filter)
    - **Receipts**: all, breakdown by payment method

    **Note**: For vendor refunds, "pending" only includes status=pending. To filter by other
    statuses (partially_allocated, allocated, completed, cancelled), use the vendor-refunds
    list endpoint with the status query parameter.

    This endpoint is optimized for efficient counting (5 queries total).