docs: update privacy policy and Play Console evidence

[jjoonleo]

Summary

• record the public account deletion and privacy policy URLs used for Google Play
• document the saved Play Console Data safety and Privacy Policy answers
• add the backend retention confirmation report and link it from the docs index

Validation

• curl -L -D - https://ontime-back.duckdns.org/privacy-policy returns HTTP/2 200 and text/html
• git diff --cached --check passed before commit

Notes

• Excluded local stray files: handoff/issue-439-frontend-deletion-verification-report.md, temp-goal.md
• Remaining release blockers are tracked in [Release] Draft and approve the privacy policy #434, [Release] Add an in-app privacy policy link #436, [Release] Complete Google Play Data safety form #441, and target audience/content in Play Console.