index car_blocks.file_id to fix FK cascade scans#665
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without this index, ON DELETE SET NULL from files.id to car_blocks.file_id causes a full-table sequential scan of car_blocks for every file being deleted. on a billion-row car_blocks table this makes file deletion effectively impossible -- the healthcheck reaper in prod wedged for 5 months because of exactly this. add gorm:"index" to CarBlock.FileID (CarID already has one). add a regression test that asserts both FK columns plus cid are indexed, so this doesn't silently regress again.
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CarBlock.FileIDwas unindexed, soON DELETE SET NULLfromfiles.idtriggered a full seq scan ofcar_blocksper deleted file. At ~1.5B rows this wedged the reaper's files phase for months.gorm:"index"onCarBlock.FileID(matchingCarID)CarID,FileID,CIDare all indexed