Persist each import's row counts (BR-34 part, ART M1) #89

Imports return total, inserted, duplicate and failed counts, but only the inserted count (row_count) is stored in csv_files. The rest are lost once the response is sent, so an import cannot be reconciled afterwards (BR-34; docs/art/milestone-0-assessment.md, M1). Scope here: persist the counts. Full reconciliation, including edited and excluded rows, is M6.

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