smrt-content: add migration and backfill from asset_associations to content_assets

[willgriffin]

Summary

We need a clean migration path for local and tenant databases that currently have content-owned rows living in asset_associations.

Scope

• Add/create content_assets where missing
• Backfill existing content-owned asset links from asset_associations into content_assets
• Decide and document whether the old rows should be deleted after backfill or left for a one-time cleanup step
• Ensure migration flow works through normal SMRT schema tooling
• Document rollout steps for local and tenant DBs

Definition Of Done

• A tenant with content-owned rows in asset_associations can migrate cleanly to content_assets
• The rollback does not require manual SQL guesswork per tenant
• Migration notes clearly explain the expected before/after state

Related

• Epic: restore base-model-owned asset join tables and remove domain reliance on asset_associations #1055
• image-donation approval fails because asset_associations UPSERT index is missing #1054