Scan multiple files

Uploads between 1 and 20 files in repeated `files` multipart parts. Each file uses its account plan's 50 MiB or 250 MiB per-file limit, validation, content-type analysis, malware detection, persistence, usage accounting, and billing path as `createScan`. The encoded multipart request limit is the plan's per-file limit × 20 plus 1 MiB for multipart framing. Results preserve submitted-file order. File-level validation or scanner failures are returned in the corresponding result and do not prevent other files from being scanned. The whole request is rejected only for request-level failures such as invalid multipart data, authentication or rate-limit failure, more than 20 files, or an oversized total request.