Legacy Ci::Pipeline doesn't belong to Ci::Ref

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Problem

We introduced Ci::Ref in !28338 (merged) that represents an object of a ref for pipelines. All pipelines must belong to at least one ci-ref, otherwise it should be considered as invalid status.

We should add an AR validation or DB constraint to make sure it's always present.

Approach

• Ship backfill migration
• Add an AR validation or DB constraint
• Wait until builds are archived
• etc