Update invoice status. ISSUED issues a draft (auto-PAID if already covered by pre-settled fees); VOID releases the claimed transactions and reverts fees to PENDING (refused if any invoice_payment rows exist). `paid` is never set directly.

Example response

{
  "success": true,
  "data": {
    "invoice_number": "INV-202605-000001",
    "payment_reference": "QK7L2M9X4P0R",
    "currency": "GBP",
    "total_amount": "1234.56",
    "amount_paid": "0.00",
    "outstanding_amount": "1234.56",
    "transaction_count": 42,
    "payments": [
      {
        "amount": "125.00",
        "currency": "GBP"
      }
    ],
    "fx_rates": [
      {
        "source_currency": "GBP",
        "rate": "0.85120000",
        "provider": "openexchangerates"
      }
    ]
  }
}