Finish a zip archive's multipart transfer and start processing

Owner only. Assembles the uploaded parts, re-validates the assembled object's actual size against MAX_ARCHIVE_BYTES, then validates the ZIP central directory via ranged R2 reads (no full-file load) and checks the entry count is between 1 and MAX_IMPORT_ENTRIES. On success, transitions the import to queued and enqueues the first processing chunk. Any validation failure here also moves the import to a terminal failed status with last_error set.