Import files by URL into durable account storage

Copies up to 20 files from public HTTP(S) URLs (for example the short-lived download link of a chat host) into the upload storage of the caller and returns durable Minds upload URLs to pass wherever a file URL is accepted (Audience briefs, Study drafts, validations, Mind knowledge, source imports). Downloads are SSRF-protected and limited to 100 MB per file; private-network URLs are refused. A file already in that storage is confirmed rather than copied. Imports are content-addressed, so retrying the same file returns the same path. All files are imported or none is reported: the first failing file is named by its one-based position ("Study asset N ...").