MNTOR-4448: merge dup subscriber script

[mansaj]

References:

Jira: MNTOR-4448

Description

This script is meant to be run as a one-off job that needs to be run in a pod. It implements the cohort-based strategy for merging duplicate subscriber entries where multiple subscribers share the same fxa_uid.
The script:

• 1. Identifies all duplicate fxa_uids
• 2. Categorizes them into 4 cohorts based on paid status and activity
• 3. Processes each cohort with appropriate merge logic
• 4. Updates related tables to point to the winning subscriber record
• 5. Performs all operations in transactions with logging

How to test

See additional testing doc / explaination

Checklist (Definition of Done)

• Commits in this PR are minimal and have descriptive commit messages.
• I've added or updated the relevant sections in readme and/or code comments
• I've added a unit test to test for potential regressions of this bug.
• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

[github-actions]

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