[core] Trigger endpoint to allow clickhouse suggest merge uid.

[Cookiezaurs]

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[Copilot]

Pull request overview

This PR introduces functionality to allow ClickHouse (or other plugins) to suggest which user ID should be the winner during user merge operations, rather than always using the default merge logic based on last session time.

Key Changes:

• Added a new plugin dispatch endpoint /i/suggest_merged_uid that allows plugins to influence merge decisions
• Modified the merge flow to handle three scenarios: winner is old UID, winner is new UID (default), or winner is a completely different UID (double merge)
• Added mergeOptions parameter to the mergeUserData function to pass plugin suggestions through to merge handlers

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
api/parts/mgmt/app_users.js	Core merge logic updated to dispatch to plugins for merge UID suggestions and handle the suggested winner UID in three different scenarios
plugins/times-of-day/tests.js	Added test to trigger mutation manager job after app reset to ensure proper cleanup

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