Public verification surface (passport + API)

[trietlu]

Description

Expose proof on the public passport and API: txHash, blockNumber, chainId, contract address, explorer link, plus fields for independent verification (watchCommitment, eventId hash, payloadHash, schemaVersion). Openness per D2 (no salt).

Affected areas

backend/src/serializers/public-passport.ts, frontend/src/app/p/[publicId]/page.tsx, native passport screen.

Dependencies / order

Depends on #10.

Acceptance criteria

• The public passport shows anchored status + an explorer link per anchored event.
• The public API returns the proof fields (owner-sensitive data still excluded).
• A documented procedure lets a third party independently verify an event against the chain.

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.