Fix concurrent map write panic in timeout middleware

- Add thread-safe header wrapper in timeoutWriter
- Check context cancellation before writing responses in handlers
- Protect header access after timeout with mutex
- Prevents race condition when requests timeout while handlers are still running

internal/server/handlers.go

@@ -557,6 +557,14 @@ func (s *Server) handleASDetail() http.HandlerFunc {
 			IPv6Count:    len(ipv6Prefixes),
 		}
 
+		// Check if context is still valid before writing response
+		select {
+		case <-r.Context().Done():
+			// Request was cancelled, don't write response
+			return
+		default:
+		}
+
 		w.Header().Set("Content-Type", "text/html; charset=utf-8")
 		tmpl := templates.ASDetailTemplate()
 		if err := tmpl.Execute(w, data); err != nil {
@@ -694,6 +702,14 @@ func (s *Server) handlePrefixDetail() http.HandlerFunc {
 			OriginCount: len(originMap),
 		}
 
+		// Check if context is still valid before writing response
+		select {
+		case <-r.Context().Done():
+			// Request was cancelled, don't write response
+			return
+		default:
+		}
+
 		w.Header().Set("Content-Type", "text/html; charset=utf-8")
 		tmpl := templates.PrefixDetailTemplate()
 		if err := tmpl.Execute(w, data); err != nil {
@@ -814,6 +830,14 @@ func (s *Server) handlePrefixLength() http.HandlerFunc {
 			"Count":      len(prefixes),
 		}
 
+		// Check if context is still valid before writing response
+		select {
+		case <-r.Context().Done():
+			// Request was cancelled, don't write response
+			return
+		default:
+		}
+
 		tmpl := template.Must(template.ParseFiles("internal/templates/prefix_length.html"))
 		if err := tmpl.Execute(w, data); err != nil {
 			s.logger.Error("Failed to render prefix length template", "error", err)