Backfill AddOn FixedItemsModel ids

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Overview

Backfill the AddOn FixedItemsModel IDs for existing records in the database. This is the data migration step that populates the new fixed model ID columns after the schema changes have been applied.

Scope

This issue implements the backfill step of the FixedItemsModel migration. Specifically:

• Run a post-migrate background migration to backfill existing records (e.g., AddOnPurchase) to reference the new fixed model IDs
• Ensure data consistency across all affected tables

Prerequisites

• Schema migration to add new FixedItemsModel ID columns must be completed
• Syncing of new IDs on write for new/updated records must be in place

Success Criteria

• All related records reference the new fixed model IDs
• Migration handles edge cases and maintains data integrity
• Tests validate the backfill process

Related Work

• Parent Epic: &19981 - Convert subscription_add_ons table to be hard-coded in application code
• POC Issue: customers-gitlab-com#15596 (closed)
• POC Branch: https://gitlab.com/gitlab-org/gitlab/-/commits/15596-add-on-fixeditemsmodel