Duplicate an asset into the caller's library.

Any asset needs only VIEW, so one shared read-only can be copied. A project is copied without its media: the copy's timeline uses the same assets. Any other file (media, stock footage, entity, config) is copied in full, stored files and analysis included, as name (1).ext next to the original when the caller can write to its folder, at the library root otherwise or with to_library_root. A folder is copied the same way with everything in it; the group clips, master clips and still-uploading files inside are left out and counted in skipped_count. Group clips, master clips, assets still uploading and folders of more than 500 items return 400. A copy whose files would not fit the caller's storage returns 413. A missing or inaccessible asset returns 404.