Rollout: staging validation then production enable

[trietlu]

Description

Enable on staging (Base Sepolia) behind BLOCKCHAIN_ENABLED, validate all event types + worker + reconciliation for a defined period, estimate mainnet gas cost, then enable production (Base Mainnet). Verify rollback (flag off).

Affected areas

env config (Vercel), docs/.

Dependencies / order

Depends on #9, #10, #11.

Acceptance criteria

• Staging runs the full flow green over the agreed validation window.
• A go/no-go checklist is documented and completed.
• Production is enabled behind the flag and validated with a real anchored event.
• Rollback verified: flag off -> new events stay NOT_REQUIRED/PENDING, no runtime errors.

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.