feat: add Public Alpha feedback link

[alteixeira20]

Summary

• add a subtle and accessible feedback link to the site footer
• direct Public Alpha feedback to the existing GitHub issue chooser
• keep vulnerability reporting separate through SECURITY.md
• document the feedback path in the changelog

Validation

• focused accessibility tests: 9 passed
• 35 frontend test files passed
• 358 frontend tests passed
• frontend lint passed
• frontend typecheck passed
• Next.js production build passed
• API lint passed
• 211 API tests passed
• Alembic reported no new upgrade operations
• git diff check passed

Roadmap

Completes RF-1203.

Remaining roadmap work consists of final manual QA, deployment verification, licensing review, and close-out reconciliation.