Bulk-import contacts from CSV

Parse a raw CSV string and upsert the rows as contacts (≤ 1000). By default each column header maps to a field of the same name (email is required); pass mapping to remap columns explicitly. Rows with a missing/invalid email are SKIPPED (counted in summary.skipped), not errored; disposable-domain rows are imported with validationStatus: "risky" (deliverable but flagged). Returns the same { summary, fieldsCreated, errors, warnings } as batch-create — 207 when some rows fail validation.