Get a bank cash match by ID

Dedicated ledger:cash staff/source authority, verified non-API-key identity, and recent step-up for staff mutations. Exact immutable content and a different reviewer are required for actual cash allocation. Every grouped journal must have the actual line direction and their sum must exactly equal that line. A line and each actual master-account journal can be consumed only once across modules. Test source statements cannot pin or allocate production journals. Signed source date remains separate from journal accounting date; this is line-source reliance, not a claim that the whole bank closing balance reconciles.