Get one transaction with its receipt and timeline context

Retrieves a single billing transaction for the transaction detail page: the full ledger record (signed amount in ten-thousandths of the currency unit, payment method, settlement status, processor reference, linked invoice, failure reason) plus, for wire transfers, the attached bank-transfer receipt metadata (file name, originating bank, IBAN, SWIFT reference, verification state). The status drives which actions the UI offers: failed exposes "Retry payment", pending-manual-approval exposes "Cancel transfer". The read does not mutate the resource. Validation: scopeId must be visible to the caller and transactionId must belong to that scope — unknown or cross-scope ids map to NOT_FOUND (the ledger never leaks other tenants' entries); the caller needs reader-or-higher or billing role, otherwise FORBIDDEN. Async & state: Synchronous read; no state change. Status transitions performed elsewhere (pay/retry/cancel, manual receipt approval) surface here and as resource.updated SSE frames with kind billing.transaction patching status, failureReason and receipt.verification. Errors: 401 unauthenticated; 403 caller lacks billing read permission; 404 unknown scope or transaction (including soft-deleted entries); 500 read-model unavailable. Idempotency/Concurrency: Safe, cacheable; no idempotency key required. The response carries no ETag; the transaction actions do not check If-Match.