perf: cache translated chapter responses for 1 hour

Translation content is immutable once generated — add Cache-Control: public,
max-age=3600, stale-while-revalidate=86400 to handleTranslationRead so the
browser and any CDN reuse the response without hitting MinIO on repeat views.
Forward the header through the SvelteKit /api/translation/[slug]/[n] proxy
which previously stripped it by constructing a bare Content-Type-only Response.

backend/internal/backend/handlers.go

@@ -1279,6 +1279,10 @@ func (s *Server) handleTranslationRead(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Translated chapter content is immutable once generated — cache aggressively.
+	// The browser and any intermediary (CDN, SvelteKit fetch cache) can reuse this
+	// response for 1 hour without hitting MinIO again.
+	w.Header().Set("Cache-Control", "public, max-age=3600, stale-while-revalidate=86400")
 	writeJSON(w, 0, map[string]string{"html": buf.String(), "lang": lang})
 }
 

ui/src/routes/api/translation/[slug]/[n]/+server.ts

@@ -23,9 +23,12 @@ export const GET: RequestHandler = async ({ params, url }) => {
 		return new Response(null, { status: res.status });
 	}
 	const data = await res.json();
-	return new Response(JSON.stringify(data), {
+	const headers: Record<string, string> = { 'Content-Type': 'application/json' };
-		headers: { 'Content-Type': 'application/json' }
+	// Forward the immutable cache header from the backend so browsers and CDNs
-	});
+	// can cache translated chapter content without hitting MinIO on every load.
+	const cc = res.headers.get('Cache-Control');
+	if (cc) headers['Cache-Control'] = cc;
+	return new Response(JSON.stringify(data), { headers });
 };
 
 export const POST: RequestHandler = async ({ params, url, locals }) => {