feat(observability): add pino request logging

[changliuchang777]

Summary

• add a pino-backed src/logger.ts with LOG_LEVEL support and test-mode silence
• replace the manual console.log(JSON.stringify(...)) request timing log with structured child logger output bound to requestId/method/path
• log unhandled request errors at error level with request correlation and stack via pino's error serializer, without logging request bodies
• configure redaction paths for Authorization and X-Api-Key style header fields
• document LOG_LEVEL and add focused logger tests

Fixes #12

Validation

• git diff --check passed except Git for Windows LF/CRLF warnings
• npm test -- --runInBand src/__tests__/logger.test.ts passed
• npx eslint src/logger.ts src/__tests__/logger.test.ts passed
• npm test -- --runInBand src/__tests__/errorHandler.test.ts blocked by existing baseline parse error in src/index.ts: duplicate WEBHOOK_MAX_EVENTS
• npm run build blocked by existing baseline TypeScript errors in src/index.ts and src/__tests__/store.adapter.test.ts
• npm run lint blocked by existing baseline lint errors unrelated to this change