QR code for the share link (public, .png alias)

Byte-identical alias of /api/files/{uuid}/qr-code whose .png extension lets clients that sniff the URL - Notion "Files & media" columns and other image-only embeds - treat it as an image. Same public access, the same per-IP limit, the same day-long cache and the same plain-text 404. Prefer it wherever a URL is pasted somewhere that may inspect the extension. The response is binary - hand the URL to the user rather than returning the bytes to a model.