Submit self-reported cashflows

Submit cashflow items for the authenticated organization. Each entry in items takes one of three shapes, selected by its type field:

• transaction — an individual transaction with a precise datetime and a required externalId for deduplication.
• aggregate — a daily total for a date (zero allowed for no-activity days).
• collateral — a cumulative collateral snapshot for a date.

Items flow through the standard capture → normalize pipeline. Transactions and aggregates become ledger entries; collateral snapshots are stored in the dedicated collateral_snapshots table.