QR code for the share link (public)

Unauthenticated QR endpoint (per-IP rate limited, 60/min). This is the URL integration outputs should embed - e.g. the Zapier qr_url output - so anything downstream can fetch the PNG without credentials. Same image and rules as the v1 twin: encodes the durable share URL with ?via=qr; unknown, expired, trashed or burned ids answer a plain-text 404 (this path is outside the /api/v1 problem-document lane). Cached for a day. The response is binary - hand the URL to the user rather than returning the bytes to a model.