feat(guard): upgrade /guard/history pagination to cursor-based system

[Pcmhacker-piro]

Summary

Closes #663

This PR replaces the offset-based pagination in the `/guard/history` endpoint with a scalable, cursor-based pagination system using a base64-encoded `(scanned_at, id)` cursor. It

enforces stable sorting (scanned_at DESC, id DESC), returns a next_cursor, and preserves/restores optional advanced filtering by decision, intent, and date ranges
(start_date, end_date). Additionally, it cleans up duplicated merge-residue code in the guard.py API module.

## Type of Change

- [x] Bug fix
- [x] New feature
- [ ] Documentation update
- [x] Refactor
- [x] Tests
- [ ] Infra / CI

## Checklist

- [x] I have read [CONTRIBUTING.md](../CONTRIBUTING.md)
- [x] My code follows the project style (PEP 8 for Python, ESLint for TS)
- [x] I have added/updated tests where relevant
- [x] `pytest backend/tests/` passes locally
- [x] I have not committed `.env` or any secrets
- [x] I have updated documentation if needed

## Screenshots (if UI change)

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[Pcmhacker-piro]

hii @SdSarthak

the checks have passed. Could you please review and approve the pending workflows when you have a chance? Thank you!