Image inputs limited to 15,000 pixels per side, larger images return 413

Image inputs now enforce a maximum width and height of 15,000 pixels per side. Requests with larger images return 413 Content Too Large with the message "Image dimensions {width}x{height} exceed the 15000px per-side limit". Existing payload-size, image-count, and decompressed-memory limits are unchanged. See Image Inputs and Errors.