Inconsistent rate limiting between v2 and v3 data-download/raw-data endpoints

# Inconsistent rate limiting between v2 and v3 data-download/raw-data endpoints

## Summary
v3 `/data-download` and `/raw-data` are rate-limited at 10/min; the v2 equivalents have no rate limiting at all. Unclear whether this gap is intentional.

## Where this came from
Baalmart, following up on AirQo-frontend PR #3876's data-download review, tracing the request path in `src/analytics`.

## Affected code
- v3 rate limiting — `views/v3/data.py:49,98`
- v2 endpoints (no rate limiting found)

## Suggested fix
Confirm which API version the nexus frontend actually calls, and either apply the same rate-limit policy to v2 or confirm the gap is intentional (e.g. v2 is deprecated/internal-only) and document it.

## Acceptance criteria
- [ ] Confirmed which version nexus calls for data-download/raw-data
- [ ] Rate limiting is consistent across v2/v3, or the discrepancy is deliberate and documented

Start with AirQo-frontend PR #3876 and trace the nexus frontend request path to confirm which API version it uses for data-download and raw-data. Compare the v3 rate limits in views/v3/data.py at lines 49 and 98 with the v2 endpoints, then verify that the final policy is either consistent across versions or explicitly documented as intentional.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.