fix: channel List reads serverID from URL param, not query string

internal/channel/handlers.go

@@ -158,7 +158,7 @@ func (h *Handler) List(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	serverID := r.URL.Query().Get("server_id")
+	serverID := chi.URLParam(r, "serverID")
 	if serverID == "" {
 		http.Error(w, `{"error":"server_id query parameter is required"}`, http.StatusBadRequest)
 		return