Fix painel polling: 304 responses, logo re-fetches, and overlapping requests

- @never_cache on get_dados_painel: ConditionalGetMiddleware was computing
  an ETag from the unchanged JsonResponse body and returning 304 with no
  body; the display always needs a live response.

- Guard logo src update: jQuery .attr("src", ...) fired a browser HTTP GET
  on every 500 ms poll even when the URL hadn't changed — 120 media
  requests/min per user, hitting the auth_threshold and triggering
  user_blocked for painel operators.

- Fix poll scheduling: setTimeout was evaluated at $.ajax options
  construction time, scheduling the next poll 500 ms after the request
  started rather than after it finished. Slow responses (>500 ms) stacked
  concurrent in-flight requests, creating a self-amplifying load loop and
  a DOM race condition where older responses could overwrite newer ones.
  Moved to a proper complete callback so at most one request is in-flight
  at any time.

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

sapl/templates/painel/index.html

@@ -284,7 +284,7 @@
                 $('#tema_solene_div').show();
               }
 
-              if (data["brasao"] != null)
+              if (data["brasao"] != null && $("#logo-painel").attr("src") !== data["brasao"])
                 $("#logo-painel").attr("src", data["brasao"]);
 
               var presentes = $("#parlamentares");
@@ -487,9 +487,9 @@
            error: function(err) {
               console.error(err);
            },
+           complete: function() { setTimeout(poll, 500); },
            dataType: "json",
-           complete: setTimeout(function() {poll()}, 500),
-           timeout: 20000 // TODO: decrease
+           timeout: 20000
         })
       })();
       });