Make transaction dispatch idempotent and reconcile unknown outcomes #798

Summary

Prevent duplicate transactions when dispatch responses are lost or the watcher retries an upload.

Required behavior

• Derive a stable idempotency key from the file ID and line identity.
• Pass the idempotency key through to the Transaction Processor.
• Make repeated delivery of the same file and line idempotent.
• Distinguish definite rejection from an unknown remote outcome.
• Reconcile timeouts before retrying.
• Persist the remote transaction ID and outcome when available.

Acceptance criteria

• Replaying the same file does not create a second transaction.
• Retrying after a timeout safely finds or reuses the original transaction.
• Tests cover lost responses, repeated submissions, restarts, and concurrent retries.

Related issues

• #793 Keep failed transaction lines eligible for retry
• #794 Pass event ID in transaction metadata
• #796 Replace listening-directory handoff with durable file storage