Complete Lab 4 request tracing and outside-in deploy debugging

[smairon]

Goal

Complete Lab 4 by documenting end-to-end request tracing and outside-in debugging of a broken QuickNotes deploy

Changes

• Added full Task 1 trace analysis in submissions/lab4.md:
  • Annotated TCP 3-way handshake, HTTP POST request, HTTP 201 response, and FIN connection close.
  • Added command outputs for listener check, routes, reachability, DNS resolution, and service logs.
  • Added a short 502 troubleshooting strategy.
• Added full Task 2 outside-in debugging in submissions/lab4.md:
  • Reproduced broken deploy by starting a second instance on :8080.
  • Captured exact bind error: listen tcp :8080: bind: address already in use.
  • Documented outside-in chain with command + output + decision for process, listener, health check, firewall, and DNS.
  • Performed repair (killed conflicting PID), restarted service, and re-verified /health response.
  • Added root cause and blameless mini-postmortem.

Testing

Verified by running the app and debugging commands directly.
Observed results:

• Broken instance failed with bind: address already in use.
• Listener and process checks identified the port owner.
• Health endpoint returned 200 after repair and json response as expected.

Checklist

• Title is a clear sentence (≤ 70 chars)
• Commits are signed (git log --show-signature)
• submissions/lab4.md updated