Add a security severity rubric for TeachLink Backend

[RUKAYAT-CODER]

Background

The Governance/ folder does not yet include a Governance/SECURITY_SEVERITY_RUBRIC.md document. This is a governance gap: contributors and maintainers lack a clear, versioned reference for this area of governance. Adding the document closes the gap and keeps the change self-contained within the Governance folder.

Implementation Plan

• Create Governance/SECURITY_SEVERITY_RUBRIC.md and document the severity levels.
• Specify the criteria per level.
• Define the response SLA per level.
• Add regression tests where applicable
• Document the change

Acceptance Criteria

• Governance requirement is implemented successfully
• Scope is limited to a maximum of two files
• No changes are made outside the Governance folder
• No regression in existing functionality
• Tests pass and code follows project standards
• Change is documented

[PHADAR6]

@PHADAR6 has applied to work on this issue as part of the Stellar Wave Program's 9th wave.

Hi maintainer I would like to work on this issue, thank you.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @PHADAR6 to this issue.

[drips-wave]

Congratulations, @PHADAR6! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on September 30, 2026.

🧑‍💻 @PHADAR6: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊