perf: add ETag and server-side caching for thumbnail proxy

Add ETag header to API thumbnail responses for 304 Not Modified support.
Forward If-None-Match/ETag through the Next.js proxy route handler and
add next.revalidate for 24h server-side fetch caching to reduce
SSR-to-API round trips on the libraries page.

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

apps/api/src/books.rs

@@ -631,12 +631,17 @@ pub async fn get_thumbnail(
         crate::pages::render_book_page_1(&state, book_id, 300, 80).await?
     };
 
+    let etag_value = format!("\"{}_{:x}\"", book_id, data.len());
+
     let mut headers = HeaderMap::new();
     headers.insert(header::CONTENT_TYPE, HeaderValue::from_static(content_type));
     headers.insert(
         header::CACHE_CONTROL,
         HeaderValue::from_static("public, max-age=31536000, immutable"),
     );
+    if let Ok(v) = HeaderValue::from_str(&etag_value) {
+        headers.insert(header::ETAG, v);
+    }
 
     Ok((StatusCode::OK, headers, Body::from(data)))
 }

apps/backoffice/app/api/books/[bookId]/thumbnail/route.ts

@@ -9,10 +9,25 @@ export async function GET(
 
   try {
     const { baseUrl, token } = config();
+    const ifNoneMatch = request.headers.get("if-none-match");
+
+    const fetchHeaders: Record<string, string> = {
+      Authorization: `Bearer ${token}`,
+    };
+    if (ifNoneMatch) {
+      fetchHeaders["If-None-Match"] = ifNoneMatch;
+    }
+
     const response = await fetch(`${baseUrl}/books/${bookId}/thumbnail`, {
-      headers: { Authorization: `Bearer ${token}` },
+      headers: fetchHeaders,
+      next: { revalidate: 86400 },
     });
 
+    // Forward 304 Not Modified as-is
+    if (response.status === 304) {
+      return new NextResponse(null, { status: 304 });
+    }
+
     if (!response.ok) {
       return new NextResponse(`Failed to fetch thumbnail: ${response.status}`, {
         status: response.status
@@ -20,13 +35,17 @@ export async function GET(
     }
 
     const contentType = response.headers.get("content-type") || "image/webp";
+    const etag = response.headers.get("etag");
 
-    return new NextResponse(response.body, {
-      headers: {
+    const headers: Record<string, string> = {
       "Content-Type": contentType,
       "Cache-Control": "public, max-age=31536000, immutable",
-      },
-    });
+    };
+    if (etag) {
+      headers["ETag"] = etag;
+    }
+
+    return new NextResponse(response.body, { headers });
   } catch (error) {
     console.error("Error fetching thumbnail:", error);
     return new NextResponse("Failed to fetch thumbnail", { status: 500 });