feat: add structured pino logging

[pq198363-ops]

Closes #37.

Summary

• add a shared pino-based src/logger.ts with LOG_LEVEL support
• default logging to silent in NODE_ENV=test and info otherwise
• redact API-key shaped fields including X-API-Key, apiKey, generated keys, and returned key values
• replace request completion and shutdown console.* calls with structured logger helpers
• keep request completion logs correlated with the existing X-Request-Id / req.id
• document LOG_LEVEL in the README

RED evidence

Before implementation, npm run build failed because src/logger.test.ts imported missing pino and ./logger.js. The focused logger contract covered:

• test-mode silence by default
• log level filtering
• API key and generated key redaction
• request completion fields including requestId
• shutdown error serialization without leaking key-shaped fields

Verification

• npx prettier --check README.md package.json src/logger.ts src/logger.test.ts src/index.ts src/middleware/index.ts
• npm run lint
• npm run build
• $env:NODE_ENV='test'; node --test dist/logger.test.js
• clean dist/, then npm run build
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js
• git diff --check

Full clean test result: 94 tests, 11 suites, all passed.

Note: the package npm test script uses POSIX-style NODE_ENV=test; on Windows PowerShell I used the equivalent build plus node --test command above.