fix: resolve all 11 Dependabot security alerts

[bakerboy448]

Summary

• Bumps all pinned dependencies to resolve all 11 open Dependabot alerts
• Flask 2.2.5 → 3.1.3 (major bump — app uses only basic routing/jsonify, no breaking changes)
• Werkzeug 3.0.2 → 3.1.6 (5 CVEs: safe_join, debugger RCE, resource exhaustion)
• Jinja2 3.1.4 → 3.1.6 (3 CVEs: sandbox breakout via filenames/format/attr)
• requests 2.32.0 → 2.32.4 (.netrc credential leak)
• Adds blinker 1.9.0 (new Flask 3.x requirement)
• Bumps itsdangerous, click, MarkupSafe to Flask 3.x compatible versions

Resolves #4, #7, #8, #10, #11, #12, #13, #14, #15, #16, #17

Test plan

• Docker image builds successfully
• /update-dns?guid=<test> endpoint responds correctly
• All 11 Dependabot alerts auto-close after merge

Summary by CodeRabbit

Release Notes

• Chores
  • Updated core framework and library dependencies to latest stable versions to improve application stability, security, and compatibility.

[coderabbitai]

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📥 Commits

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📒 Files selected for processing (1)

• requirements.txt

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✨ Finishing Touches

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• Create PR with unit tests
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• Commit unit tests in branch fix/dependabot-vulnerabilities

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