centralize API error handling across all backend services

[Jessepriase]

• Add centralized error handler (error-handler.ts) with ApiError class and handleApiError utility
• Replace all raw res.writeHead/res.end error responses with sendErr/handleApiError
• Replace all raw res.writeHead success responses with sendOk
• Fix duplicate/broken route handlers in events-server.ts
• Update rate-limiter.ts to use sendErr for 429 responses
• Ensure all endpoints return consistent error response format

Overview

Related Issue

Closes #

Changes

Verification

# e.g.
cd dashboard && npm test
cd listener && npm run typecheck && npm test
cd contract/contracts/hello-world && cargo test

How to Test

Checklist

• Branch is up to date with main
• Tests added/updated and all pass locally
• cargo fmt --all run (if Rust changes)
• npm run lint passes (if TypeScript changes)
• Documentation updated if behavior changed
  closes Centralize API Error Handling #381