Deduplicate subscriptions in GET /subscription

[J164]

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[Copilot]

Pull Request Overview

This PR modifies the GET /subscription endpoint to deduplicate subscriptions by mailing list, aggregating user IDs for each unique mailing list instead of returning individual subscription records.

• Implemented aggregation logic to group subscriptions by mailing list with arrays of user IDs
• Updated the response format from individual subscription objects to aggregated objects with mailingList and userIds properties
• Added comprehensive test coverage for the new aggregation behavior

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/services/subscription/subscription-router.ts	Added reduce operation to aggregate subscriptions by mailing list
src/services/subscription/subscription-router.test.ts	Updated existing test and added new test for multiple mailing lists aggregation

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