feat(auth): add GDPR-ready PII export and account deletion workflows

[yuliuyi717-ux]

Summary
Implements Issue #76 by adding authenticated self-service data export and irreversible account deletion flows, with audit logging and backend test coverage.

What changed

• Added GET /auth/export-data and GET /auth/export for authenticated PII export.
• Export payload now includes profile, categories, expenses, recurring expenses, bills, reminders, subscriptions, ad impressions, and audit logs.
• Added DELETE /auth/delete-account and DELETE /auth/delete for irreversible account deletion.
• Added explicit delete confirmation checks via confirm=true, confirmed=true, confirmation="DELETE", or query confirm.
• Added audit actions USER_DATA_EXPORTED and USER_ACCOUNT_DELETED.
• Added helper serializers for dates/enums and model-to-dict mapping for export response consistency.
• Added refresh-session revocation on deletion by removing matching refresh-token keys from Redis.
• Deletion flow now removes user-owned records, anonymizes existing audit rows (user_id -> NULL), and deletes the user account.
• Updated backend auth tests in packages/backend/tests/test_auth.py and added implementation report in AUTODEV_REPORT.md.

Validation done

• cd packages/backend && REDIS_URL=redis://localhost:6379/15 PYTHONPATH=. ../../.venv/bin/pytest tests/test_auth.py -q (5 passed)
• cd packages/backend && REDIS_URL=redis://localhost:6379/15 PYTHONPATH=. ../../.venv/bin/pytest tests -q (24 passed)
• ./.venv/bin/flake8 packages/backend/app/routes/auth.py packages/backend/tests/test_auth.py (passed)

Risks/notes

• Test/runtime execution depends on Redis availability.
• Existing access JWTs issued before deletion are not centrally revoked; refresh sessions are revoked.
• Export currently returns JSON directly; large-user exports may later need async generation/streaming.

[yuliuyi717-ux]

Hi maintainers, just a friendly follow-up on this PR for GDPR-ready PII export and account deletion. When you have time, could you please take a look and share any feedback? Happy to make changes quickly.