fix: handle upstream photo fetch failures in demo endpoints

[AlexBill01]

Handle upstream photo fetch failures in demo endpoints

Description

Problem

The photo demo endpoints fetch picsum.photos after the MPP payment gate passes, but the fetch is never guarded:

if (result.status === 402) return result.challenge;

const res = await fetch("https://picsum.photos/1024/1024");
const url = res.url;

return result.withReceipt(Response.json({ url }));

If the upstream image host fails (timeout, 5xx, DNS/network error), the unhandled rejection surfaces as an opaque 500. Because the failure happens before withReceipt, no receipt is issued either, leaving a paid request in an ambiguous state with no actionable error.

Fix

Wrap the upstream fetch in try/catch, also checking res.ok. On failure, log and return a clear 502 Bad Gateway without issuing a receipt — so payment is not finalized for content that could not be delivered:

let url: string;
try {
  const res = await fetch("https://picsum.photos/1024/1024");
  if (!res.ok) throw new Error(`upstream responded ${res.status}`);
  url = res.url;
} catch (error) {
  console.error("[photo] upstream fetch failed:", error);
  return Response.json(
    { error: "Failed to load photo from upstream" },
    { status: 502 },
  );
}

The same guard is applied to all four photo endpoints (api/photo.ts, api/sessions/photo.ts, api/payment-link/photo.ts, api/payment-link/photo-stripe.ts). The HTML endpoints return a plain-text 502 body instead of JSON.

[vercel]

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