Support region query parameter filtering via study_region

[zigzagdev]

Description
Summary

Add support for the region query parameter so that requests can be filtered by the study_region column.

If a valid region query parameter is provided, the API should return only records whose study_region matches the requested region.
If the region query parameter is not provided, the API should continue returning records from all regions.

Goal

Ensure the API can accept requests containing a region query parameter and return a correct response based on the backend contract.

Scope

accept region as a query parameter

validate and map the incoming region value

apply filtering by study_region when region is present

keep existing behaviour unchanged when region is absent

cover the behaviour with request / feature tests

Expected behaviour

GET /...?...&region=AFR → returns only records whose study_region is AFR

GET /... without region → returns records from all regions

invalid region values should be handled according to the existing validation policy

Notes

This issue is focused on backend support for the region query parameter and the filtering contract around study_region