database/scanned_prompts.py links a file to the prompt of one text, creating
it when new, inside one BEGIN IMMEDIATE transaction: a file linked to
another prompt is a conflict, a failed link leaves no prompt behind and two
writers store one prompt. pair_ingestion gains file_link_rows,
link_conflict and Marks/mark_created; ingest_resolved_pair takes optional
marks for the components it creates. recorded_runs and named_prompt in
run_attribution become public (#247).