Create vendor refund

    Create a new vendor refund record (parent transaction).

    **Idempotency**: Uses `external_id` field - duplicate requests with the same
    `external_id` will return HTTP 200 with the existing refund instead of creating
    a duplicate.

    **Required Fields:**
    - `vendor_id`: UUID of the vendor issuing the refund
    - `total_refunded_amount_cents`: Total amount being refunded in cents (e.g., 10050 for £100.50)
    - `currency`: ISO 4217 currency code (EUR, GBP, USD)

    **Optional Fields:**
    - `external_id`: Your system's unique identifier (for idempotency)
    - `reference_number`: External reference number
    - `refund_type`: Type of refund (overpayment, billing_error, goods_return, etc.)
    - `refund_date`: Date of refund (defaults to today)
    - `description`: Detailed description
    - `memo`: Internal memo/notes
    - `reason`: Reason for the refund
    - `metadata`: Additional structured data (JSON object)
    - `allocations`: Array of allocations to create with the refund
    - `payments`: Array of payments to create with the refund
    - `document`: Optional document upload (multipart field `file` with optional document_type, upload_source, metadata, tags)

    **Common Scenarios:**
    1. **Overpayment refund** - Vendor returns excess payment
       ```json
       {
         "vendor_id": "uuid",
         "total_refunded_amount_cents": 15000,
         "currency": "GBP",
         "refund_type": "overpayment",
         "description": "Refund of overpayment"
       }
       ```

    2. **Billing error** - Vendor refunds due to billing mistake
       ```json
       {
         "vendor_id": "uuid",
         "total_refunded_amount_cents": 50000,
         "currency": "GBP",
         "refund_type": "billing_error",
         "reference_number": "REF-12345",
         "allocations": [{
           "allocation_type": "bill",
           "bill_id": "bill-uuid",
           "amount_cents": 50000
         }]
       }
       ```

    3. **Complete refund with payment** - Create refund with allocation and payment in one request
       ```json
       {
         "vendor_id": "uuid",
         "total_refunded_amount_cents": 100000,
         "currency": "GBP",
         "external_id": "my-refund-123",
         "allocations": [{
           "allocation_type": "bill",
           "bill_id": "bill-uuid",
           "amount_cents": 100000
         }],
         "payments": [{
           "refunded_amount_cents": 100000,
           "payment_method": "bank_transfer",
           "processor": "wise"
         }]
       }
       ```

    **Returns:**
    - HTTP 201: Refund created successfully
    - HTTP 200: Existing refund (idempotent - same external_id)
    - HTTP 400: Validation error