Create a business event and post its journal entry

Strict by default (#1705/#1790/#1815): a request that cannot produce a journal entry — unknown event type, unresolvable accounts, or a resolve-time posting blocker (closed or missing period, filing-locked period, locked/non-postable account, out-of-domain date) — returns 422 naming the blocker, before any row is written. Pass allowWithoutJournalEntry: true to save the event with jeGenerationSkipped instead and retry-post after fixing the blocker. A client idempotencyKey identifies one create operation within this company. An identical completed request returns its existing event with 200 and idempotentReplay: true; this never posts again. A changed request or unfinished/uncertain create returns 409. Safely skipped stored outcomes remain visible on replay. A keyed posting or schedule failure returns 409 even on its first response and stays held for review because its write may already have committed. Exception (#2529): when a vendor bill's journal entry posts but its Payables bill cannot be written, the entry is voided and the outcome is known, so the create completes with 201 and jeGenerationError naming the reason; the event stays listed and can be retried or deleted. Vendor bills (#2529) also return 422 field errors for parameters.apItemId (must be a UUID), parameters.dueDate (a real YYYY-MM-DD date, not before the bill date) and parameters.createNewApItem (boolean; not together with apItemId).