Reconcile stuck SUBMITTED events

[trietlu]

Description

Recover events left SUBMITTED (process died between submit and receipt). Reconcile by stored txHash if present, else by scanning ProofAnchored logs keyed by the eventId hash (sha256(eventUid)); converge to ANCHORED or re-queue.

Affected areas

backend/src/services/onchain.service.ts, the cron worker.

Dependencies / order

Depends on #8.

Acceptance criteria

• An event left SUBMITTED whose tx actually landed is reconciled to ANCHORED without re-submitting.
• An event left SUBMITTED with no landed tx is re-queued to PENDING/FAILED_RETRYABLE.
• Covered by a test or a documented simulation.

Definition of done

• Implemented on a short-lived feature branch, merged into staging (never main).
• Verified on the staging preview stack.
• Docs updated if infra/env/API behavior changes.