Add STRIDE threat model to security docs

[aclark4life]

Changes proposed in this pull request:

• Update .github/SECURITY.md with threat model summary and link to handbook
• Add docs/handbook/security.rst with full STRIDE analysis (14 threats across Spoofing, Tampering, Repudiation, Information Disclosure, Denial of Service, and Elevation of Privilege categories)
• Add prioritised mitigation recommendations
• Link security.rst into the handbook toctree

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a STRIDE-based threat model to the Pillow security documentation and links it from the handbook and GitHub Security Policy to help downstream integrators assess and mitigate risks when processing untrusted images.

Changes:

• Introduces a new docs/handbook/security.rst page with STRIDE threats and prioritized mitigations.
• Adds the security page to the handbook toctree.
• Updates .github/SECURITY.md with a threat model summary and link to the handbook page.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
docs/handbook/security.rst	New STRIDE threat model and prioritized mitigation recommendations.
docs/handbook/index.rst	Links the new security page into the handbook navigation.
.github/SECURITY.md	Adds a threat model summary and links to the full handbook page.

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