Split the operation via SeparateDatabaseAndState instead of adding the ForeignKey in one AddField on an existing table.

Pre-creating an index with AddIndexConcurrently is no longer treated as sufficient to silence R006. It can help query plans, but the AddField(ForeignKey) operation still couples column creation, implicit index/constraint work, and FK validation into one deployment step.

A safer rollout is: add the column without the FK constraint, backfill if needed, build supporting indexes concurrently, then add/validate the constraint separately while keeping Django state aligned.
