Uploads previously went through a single pwg.images.addSimple multipart
POST, so files exceeding the target's upload_max_filesize/post_max_size
failed outright. Splitting each download into 500KB chunks (matching
core's own upload_form_chunk_size default) and sending them via
pwg.images.uploadAsync means uploads now succeed regardless of the
target's PHP upload limits.

uploadAsync takes tag_ids instead of addSimple's tag names, so tags are
now resolved/created on the target via pwg.tags.add before upload.