[BACKEND] Add request logging middleware with env-based verbosity

[Trovic1]

Summary

Log method, path, status, and duration per request, controlled by a LOG_LEVEL env var.

Scope

• backend only

Likely files

• src/app.js
• src/middleware/ (new logger)

Acceptance criteria

• Logs each request with status and latency
• Verbosity controlled by env var
• Silent in test mode

Verification

• Hit endpoints and confirm log lines; set log level to off and confirm silence

Labels: backend, enhancement, good first issue