perf: stream Stripstream images and increase image fetch timeout

Stream image responses directly to the client instead of buffering the
entire image in memory, reducing perceived latency. Increase image fetch
timeout from 15s to 60s to avoid AbortError on slow page loads.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/api/stripstream/images/books/[bookId]/pages/[pageNumber]/route.ts

@@ -35,14 +35,15 @@ export async function GET(
     const response = await client.fetchImage(path);
 
     const contentType = response.headers.get("content-type") ?? "image/jpeg";
-    const buffer = await response.arrayBuffer();
+    const contentLength = response.headers.get("content-length");
 
-    return new NextResponse(buffer, {
-      headers: {
-        "Content-Type": contentType,
-        "Cache-Control": "public, max-age=86400",
-      },
-    });
+    const headers: Record<string, string> = {
+      "Content-Type": contentType,
+      "Cache-Control": "public, max-age=86400",
+    };
+    if (contentLength) headers["Content-Length"] = contentLength;
+
+    return new NextResponse(response.body, { headers });
   } catch (error) {
     logger.error({ err: error }, "Stripstream page fetch error");