chore(server): improve migration compatibility (#15014)

#### PR Dependency Tree


* **PR #15014** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

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## Summary by CodeRabbit

* **Bug Fixes**
* Remove orphaned legacy subscription and entitlement records during
backfill.
* Repair workspaces missing active owners by promoting eligible members
and cleaning up empty workspaces.
* Skip cloud subscription backfill when target user/workspace no longer
exists to avoid dangling data.

* **Tests**
  * Added tests verifying legacy data cleanup during backfill.
* Added tests verifying workspace ownership repair and migration
behavior.

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