durable async execution (idempotency, retries, DLQ, replay)

[ashupednekar]

Summary

Add durable execution semantics for async invocations (idempotency, retries, DLQ, replay).

Motivation

Current async flow lacks strong guarantees for duplicate suppression, retry behavior, and operator-driven recovery.

Proposal

• Add idempotency key support at ingestor entrypoints.
• Define retry policy (max attempts, backoff, terminal failure).
• Add dead-letter subject/stream per project.
• Add replay tool/API for DLQ messages.

Acceptance criteria

• Duplicate requests with same key are deduplicated within configured window.
• Failed async invocations are retried and then moved to DLQ.
• DLQ replay path is documented and tested.