feat(realtime): long polling for app-wide events behind proxies

When a proxy lets neither WebSocket nor SSE through, the app opens a
poll connection: the server buffers its events (an SSEClient without a
stream), each GET waits up to 25 s, and room joins work like for SSE.
Tasks and lists now update live there too, and note sync takes its
notifications from it instead of a second long poll. Idle connections
close after two minutes. The profile shows such a connection as limited.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/__tests__/realtime.test.ts

@@ -137,4 +137,36 @@ describe('realtime fallback', () => {
     expect(FakeEventSource.instances).toHaveLength(1); // no reconnect loop: note sync uses long polling
     api.disconnectWebSocket();
   });
+
+  it('behind such a proxy, events come by long polling', async () => {
+    sessionStorage.setItem('mindsift_ws_blocked', 'true');
+    sessionStorage.setItem('mindsift_sse_buffered', 'true');
+    let polls = 0;
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async (url: string, init?: RequestInit) => {
+        if (url === '/api/v1/events/poll') return new Response('{"pollId":"poll_1"}', { status: 200 });
+        if (url.startsWith('/api/v1/events/poll/poll_1')) {
+          polls++;
+          if (polls === 1) return new Response('{"events":[{"event":"TASK_UPDATED","payload":{"id":"t1"}}]}', { status: 200 });
+          // Later polls wait like the server does, until the client gives up on them
+          return new Promise<Response>((_, reject) => init?.signal?.addEventListener('abort', () => reject(new Error('aborted'))));
+        }
+        return new Response('{}', { status: 200 });
+      })
+    );
+    const api = await loadApi();
+    const seen: string[] = [];
+    api.onWebSocketMessage((m) => seen.push(m.event));
+    await api.connectWebSocket();
+    await vi.advanceTimersByTimeAsync(100);
+
+    expect(FakeEventSource.instances).toHaveLength(0);
+    expect(api.isRealtimeConnected()).toBe(true);
+    expect(api.getRealtimeTransport()).toBe('poll');
+    expect(seen).toEqual(['TASK_UPDATED']);
+    expect(polls).toBe(2); // the next poll is waiting
+    api.disconnectWebSocket();
+    expect(api.isRealtimeConnected()).toBe(false);
+  });
 });