Remove unnecessary opensearch client initializations

[epipav]

Changes proposed ✍️

What

🤖[deprecated] Generated by Copilot at cc8a366

This pull request refactors the code in the search_sync_api service to use a common OpenSearchService instance from the request object. This reduces the number of unnecessary connections to the OpenSearch cluster and simplifies the code in the activity.ts, member.ts, and organization.ts files.
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🤖[deprecated] Generated by Copilot at cc8a366

To improve the code's performance and look
The devs refactored how they used opensearch
They passed it as a property of req
Instead of making new instances, you see
And thus they reduced the code's complexity

Why

How

🤖[deprecated] Generated by Copilot at cc8a366

• Simplify the code by using req.opensearch instead of creating new OpenSearchService instances in various routes (link, link, link, link, link, link, link, link, link, link, link, link, link, link) in activity.ts, member.ts, and organization.ts

Checklist ✅

• Label appropriately with Feature, Improvement, or Bug.
• Add screenshots to the PR description for relevant FE changes
• New backend functionality has been unit-tested.
• API documentation has been updated (if necessary) (see docs on API documentation).
• Quality standards are met.