#629 feat(backfill): include artist_id in session migration

This update modifies the migrateRoom function to include the artist_id when inserting a session. Additionally, the test suite for migrateRoom has been enhanced to verify that the artist_id is correctly passed to the insertSession function, ensuring that the migration process captures this important data point for each room. All related tests have been updated to reflect these changes.

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Summary by cubic

Include artist_id when inserting sessions in migrateRoom to preserve artist attribution during backfill. Tests verify insertSession gets artist_id (null when absent; set for straggler rooms), with a simplified one-line assertion.