feat: PII export, deletion workflow and audit trail

[Moyuchiiii]

Summary

Fixes #76

Adds privacy/GDPR endpoints under /privacy:

• GET /export - Export all user PII as JSON
• POST /delete-request - Initiate 30-day grace period deletion
• POST /delete-cancel - Cancel pending deletion
• POST /delete-confirm - Permanently delete all user data
• GET /deletion-status - Check deletion status
• GET /audit-log - View audit trail

Extended AuditLog model with ip_address, user_agent, detail fields.
Added deletion_requested_at and deletion_scheduled_for to User model.
Audit logs are preserved after user deletion (user_id nullified) for compliance.

Includes 8 test cases and schema migration support.

---

Auto-generated by bounty-hunter

[Moyuchiiii]

Hi @rohitdash08! Just following up — would appreciate it if you could take a look when you get a chance. Let me know if anything needs to be adjusted. Thanks!