docs(lab4): complete network debugging

[Mysteri0K1ng]

Goal

Complete Lab 4 network tracing, outside-in debugging, and TLS handshake analysis.

Changes

• Added submissions/lab4.md
• Added annotated TCP/HTTP packet trace for POST /notes
• Documented debugging commands using ss, ip route, mtr, dig, and journalctl
• Reproduced and diagnosed a broken deploy caused by bind: address already in use
• Added repair verification and a blameless mini-postmortem
• Added bonus TLS handshake evidence with ClientHello, ServerHello, and certificate chain screenshots

Testing

Verified QuickNotes locally through /health, captured and decoded a real POST /notes request with tcpdump, reproduced the port conflict failure, repaired the service, and verified HTTPS through Caddy on localhost:8443.

Checklist

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