Returns a single transaction by its accounting-ledger id (accounting_ledger_tx_id). The
response additionally includes the attached transaction_labels, merchant_details and
payables (the latter two only for banking_bank_transactions, and payables only when the
Pay product is active).

Note: show returns a different, narrower projection than the list endpoint. It does
not compute the labeled ledger fields (transaction_type, status, charge, deposit,
amount, exchange_rate, target_cumulative_balance_with_commission, commission fields,
etc.) that GET /v1/transactions adds — those are list/CSV-only. The plain
cumulative_balance / target_cumulative_balance keys are the exception: they are virtual
attributes aliased to the persisted ledger value, so they are present on both the list and
the show responses. See the response schema.