Upload a CSV file containing the vendor's product list to match products against the master catalog and create vendor products from the matches.

Two match paths:

• Deterministic (round-trip) — When a row has a populated master_product_id, the row is
  resolved directly to that master product. If the ID is unknown, unpublished, archived, or
  non-numeric, the row fails with an explicit error — there is no fuzzy fallback.

• Fuzzy (template-download) — When master_product_id is blank, the row is matched by
  name + brand against the catalog. Matches with confidence >= 0.95 are auto-linked;
  others are flagged for manual resolution.

Required CSV columns: name, sku, price, stock_level
Optional CSV columns: master_product_id, brand, category, currency, discount_percentage
The category column is accepted but ignored — vendors cannot change a master product's category. Any extra columns from older exports (e.g. status) are also silently ignored; product status is controlled by the form's default_status field, which applies to every row in the upload.
Both formats are accepted: the legacy 5-column template (name,brand,sku,price,stock_level) and the canonical 7-column shape that matches the export (master_product_id,name,brand,category,sku,price,stock_level).

The import runs asynchronously. Use the returned bulk_upload_id to poll for status.