Finalize WatchRegistry contract + Hardhat tests

[trietlu]

Description

Review WatchRegistry.sol; confirm the on-chain eventType numbering matches EVENT_TYPE_ID in onchain.service.ts; add NatSpec. Remove recordEvent (decision D1 — backend only uses anchorProof). Enable a real contract test suite.

Affected areas

contracts/contracts/WatchRegistry.sol, contracts/test/, contracts/package.json.

Dependencies / order

First task; nothing blocks it.

Acceptance criteria

• recordEvent removed; anchorProof retained.
• Hardhat tests cover: successful anchorProof, Unauthorized revert for a non-writer, and setAuthorizedWriter toggling access.
• npx hardhat test passes and the contracts test script runs it (no longer a stub).
• The eventType enum is documented and matches EVENT_TYPE_ID exactly.

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.