docs(lab2): add submission2 threat model

[idio4]

Goal

Merge Lab 2 work into my fork main branch.

Changes

• Added labs/submission2.md with baseline and secure Threagile analysis.
• Added baseline artifacts in labs/lab2/baseline/.
• Added secure artifacts in labs/lab2/secure/.
• Added secure model labs/lab2/threagile-model.secure.yaml.

Testing

• Ran Threagile baseline generation from labs/lab2/threagile-model.yaml.
• Ran Threagile secure generation from labs/lab2/threagile-model.secure.yaml.
• Verified output files: report.pdf, diagrams, risks.json, stats.json, technical-assets.json.
• Ran risk delta comparison (Baseline vs Secure).

Artifacts & Screenshots

• labs/submission2.md
• labs/lab2/baseline/report.pdf
• labs/lab2/secure/report.pdf
• labs/lab2/baseline/data-flow-diagram.png
• labs/lab2/secure/data-flow-diagram.png

Checklist

• Title is clear and specific
• Docs updated if needed
• No secrets or large temp files committed