feat: preserve Express errors in automatic logs

[drewmt]

Summary

• add an Express error middleware to preserve the original application error on the response
• forward the same error to the remaining Express error handlers without registering duplicate log listeners
• document the middleware ordering and cover the runtime and TypeScript contracts

Why

Express response completion events do not expose an error after the application error handler has processed it. This lets pino-http use the original error instead of synthesizing a generic status-code error for automatic logs.

Testing

• npm test
• 65 tests passed, 1 existing test skipped
• 100% statement, branch, function, and line coverage for logger.js
• TypeScript, tsd, and ts-node validation passed

Closes #348