docs: add comprehensive security review documentation

[esmcelroy]

Summary

Adds comprehensive security review documentation covering threat modeling, authentication, authorization, and data protection.

Changes

• SECURITY_REVIEW.md: Complete security analysis with threat model
• SECURITY_REVIEW_INDEX.md: Quick reference index for navigation
• SECURITY_REVIEW_SUMMARY.md: Executive summary of findings
• SECURITY_REVIEW_COMPLETION_REPORT.md: Detailed findings and status
• SECURITY_IMPLEMENTATION_GUIDE.md: Implementation guidelines and best practices

Documentation Covers

• Authentication & authorization (JWT, bcrypt)
• Data protection & validation
• API security & CORS
• Input sanitization
• Error handling
• Security best practices for Express backend
• React frontend security considerations

Value

Provides security baseline and guidelines for:

• Current implementation review
• Future feature development
• Security audit preparation
• Onboarding new developers