Promote Due Schedule Obligations

Promote matured pending schedule obligations (schedule_entry_due events whose period boundary has passed) to 'classified', and — when dispatch_handlers=true (default) — draft their closing entries in the same transaction. Also reaches stranded obligations: events already 'classified' (by an earlier flip-only sweep) whose closing entry was never drafted are dispatched in the same pass, and reported via stranded_count. This is the on-demand form of the background obligation-promotion sweep; run it before close-period when a schedule was just created or when you can't wait for the Dagster sensor. Idempotent: re-running skips already-classified obligations and reconciles to existing drafts.

Idempotency: supply an Idempotency-Key header to make safe retries; replays within 24 hours return the same envelope. Reusing the key with a different body returns HTTP 409 Conflict.