Fix/cursor pagination

[sakeena-7878]

Summary

Closes #663
This PR replaces offset-based pagination in the LLM Guard /history endpoint with cursor-based pagination using a base64-encoded (scanned_at, id) cursor.

A dedicated CursorPaginatedResponse schema is introduced to avoid breaking the existing PaginatedResponse contract used elsewhere in the system.
SQLite compatibility handled (no tuple comparison operator used; logic uses explicit and_/or_ filtering)
Frontend has been updated to support cursor-based pagination instead of page/offset.

Type of Change

• Bug fix
• New feature
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My code follows the project style (PEP 8 for Python, ESLint for TS)
• pytest backend/tests/ passes locally
• I have not committed .env or any secrets

Screenshots (if UI change)

Replaced page/limit pagination with cursor/next_cursor

[SdSarthak]

Two regressions need to be fixed before this can merge:

1. The empty-prompts validation was removed (if not self.prompts: raise ValueError). Batch requests with zero prompts will now silently succeed with an empty response — please restore that guard.
2. db: Session was removed from scan_prompt along with the webhook delivery block. That coupling was intentional — the webhook trigger relies on the db session. If the goal is to separate concerns, use BackgroundTasks to deliver the webhook asynchronously, but don't drop it entirely.

The CursorPagination helper itself and the CursorPaginatedResponse schema are the right direction — just fix the two regressions above.

[sakeena-7878]

Ok got it

[SdSarthak]

The two regression fixes are in — the empty-prompts guard is restored and the db: Session + webhook delivery block are back. The implementation now looks correct. Please rebase on main to resolve the current conflicts and it will be ready to merge.

[sakeena-7878]

This PR resolves guard module merge conflicts and completes the cursor-based pagination refactor.

Key updates:

• Backend: cleaned guard endpoints, fixed schema imports, resolved pagination logic conflicts
• Frontend: fixed API typing issues (RagSource, Guard interfaces), removed duplicates
• Lint/build: frontend passes lint and production build successfully
• Tests: updated and passing for guard API and related modules

Please let me know if any adjustments are needed.