Upload a BOM file and read it into reviewable lines

Multipart upload (file, optional userPrompt of at most 500 characters) of an xlsx, csv, pdf or image file of at most 10 MiB. The AI gateway extracts the line items (name, quantity, unit, stage) and each line is matched against the PCF factor library; the call waits for both. The import is returned with status ready, or error when extraction failed (errorMessage says why; reanalyze retries). Nothing is written to the BOM until the import is applied. 503 AI_GATEWAY_UNAVAILABLE when the gateway is not configured. Requires org.write_data.