Add background migration to remove duplicates from push rules table

What does this MR do and why?

Contributes to #493391

Problem

Push rules table includes duplicated records with the same project_id. It's incorrect because it must have only one record per project.

Solution

Add a background migration to identify and remove duplicates.

References

Push rules: add a unique index by project_id (#493391)

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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Related to #493391