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})
 }