Anchoring worker (Vercel Cron): retry with backoff + FINAL escalation

[trietlu]

Description

Implement a Vercel Cron worker (decision D3) that claims PENDING/FAILED_RETRYABLE events and (re)submits via anchorProof, with: exponential backoff, an attempt cap -> FAILED_FINAL, retryable-vs-final classification (network/nonce/underpriced = retryable; revert/unauthorized = final), and idempotency (never double-submit one event).

Affected areas

backend/src/services/onchain.service.ts, a new cron route under backend/api/, backend/vercel.json (cron schedule).

Dependencies / order

Depends on #7.

Acceptance criteria

• A simulated transient failure is retried and eventually reaches ANCHORED.
• Exceeding the attempt cap sets FAILED_FINAL with anchorError populated.
• No event produces two on-chain ProofAnchored records.
• The Vercel Cron schedule is configured in vercel.json and documented.

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.