feat(notes): push, long polling and fallback detection for note sync

Phase 4 of the collaboration plan:
- The sync engine listens for DOC_SEQ in the note's realtime room and syncs
  right away. Without WebSocket/SSE it long-polls /docs/{id}/changes (25 s);
  after three failures it falls back to polling every 3 s and retries long
  polling when the browser comes back online.
- With push or long polling the engine only syncs every 7 s while holding
  leases (renewal), otherwise every 30 s as a safety net. Focusing a block
  announces its lease immediately (otherwise it could take up to 30 s).
- Realtime connection: SSE counts as connected only after its first event;
  a stream that stays silent for 5 s (buffering proxy) switches SSE off for
  the tab. App-level PING/PONG detects WebSockets that are accepted but
  forward nothing and switches to SSE after 35 s.
- The editor shows the active channel for shared notes.
- Tests: engine channel tests (push, heartbeat, lease on focus, long
  polling, interval fallback, switching), realtime fallback tests with fake
  WebSocket/EventSource, Go tests for PONG and DOC_SEQ delivery, and
  e2e/channels.e2e.mjs measuring latency behind six simulated proxy setups
  (0.2-0.3 s on all live channels, ~3 s with plain polling).

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

frontend/src/lib/api.ts

@@ -1134,6 +1134,16 @@ export function onRealtimeStatus(callback: (connected: boolean, transport: Realt
 }
 
 let isWsBlockedByProxy = typeof window !== 'undefined' && sessionStorage.getItem('mindsift_ws_blocked') === 'true';
+// Some proxies buffer event streams: the connection opens, but events only arrive much later (or never).
+// Then SSE is useless and the note sync engines fall back to long polling.
+let isSseBufferedByProxy = typeof window !== 'undefined' && sessionStorage.getItem('mindsift_sse_buffered') === 'true';
+let sseFirstEventWatchdog: any = null;
+const SSE_FIRST_EVENT_TIMEOUT_MS = 5000;
+// Some proxies accept the WebSocket upgrade but then forward nothing. An app-level ping detects that.
+let wsHeartbeatTimer: any = null;
+let wsLastMessageAt = 0;
+const WS_PING_INTERVAL_MS = 15000;
+const WS_DEAD_AFTER_MS = 35000;
 let sseReconnectTimer: any = null;
 let sseReconnectDelay = 2000;
 const globalSubscribedRooms = new Set<string>();
@@ -1153,6 +1163,7 @@ function notifyRealtimeStatusChange(connected: boolean, transport: RealtimeTrans
 
 function switchToSSE(reason: string) {
   console.warn(`[MindSift Realtime] ${reason} -> Schalte sofort und dauerhaft auf Server-Sent Events (SSE) um.`);
+  clearInterval(wsHeartbeatTimer);
   isWsBlockedByProxy = true;
   try {
     sessionStorage.setItem('mindsift_ws_blocked', 'true');
@@ -1177,6 +1188,12 @@ export function connectSSE(rooms?: string[]): void {
   clearTimeout(sseReconnectTimer);
   clearTimeout(wsReconnectTimer);
   clearTimeout(wsConnectionWatchdog);
+  clearTimeout(sseFirstEventWatchdog);
+
+  if (isSseBufferedByProxy) {
+    notifyRealtimeStatusChange(false, 'none');
+    return;
+  }
 
   if (activeGlobalSSE) {
     try { activeGlobalSSE.close(); } catch {}
@@ -1190,18 +1207,28 @@ export function connectSSE(rooms?: string[]): void {
 
     es.onopen = () => {
       sseReconnectDelay = 2000;
-      notifyRealtimeStatusChange(true, 'sse');
-      if (activeSSEId && globalSubscribedRooms.size > 0) {
-        globalSubscribedRooms.forEach((room) => {
-          api.joinSSERoom(activeSSEId!, room, 'join').catch(() => {});
-        });
-      }
+      // Connected only counts once the first event really arrives (see SSE_CONNECTED below)
+      clearTimeout(sseFirstEventWatchdog);
+      sseFirstEventWatchdog = setTimeout(() => {
+        console.warn('[MindSift Realtime] SSE liefert keine Ereignisse (Proxy puffert) -> Long-Polling');
+        isSseBufferedByProxy = true;
+        try {
+          sessionStorage.setItem('mindsift_sse_buffered', 'true');
+        } catch {}
+        try {
+          es.close();
+        } catch {}
+        if (activeGlobalSSE === es) activeGlobalSSE = null;
+        notifyRealtimeStatusChange(false, 'none');
+      }, SSE_FIRST_EVENT_TIMEOUT_MS);
     };
 
     es.onmessage = (event) => {
       try {
         const msg = JSON.parse(event.data) as WSMessage;
         if (msg.event === 'SSE_CONNECTED') {
+          clearTimeout(sseFirstEventWatchdog);
+          notifyRealtimeStatusChange(true, 'sse');
           activeSSEId = msg.payload?.sseId || null;
           if (activeSSEId && globalSubscribedRooms.size > 0) {
             globalSubscribedRooms.forEach((room) => {
@@ -1223,6 +1250,7 @@ export function connectSSE(rooms?: string[]): void {
     };
 
     es.onerror = (e) => {
+      clearTimeout(sseFirstEventWatchdog);
       console.warn('[MindSift Realtime] SSE Verbindungsfehler:', e);
       if (activeTransport === 'sse') {
         notifyRealtimeStatusChange(false, 'none');
@@ -1279,6 +1307,17 @@ export function connectWebSocket(): void {
 
     ws.onopen = () => {
       clearTimeout(wsConnectionWatchdog);
+      wsLastMessageAt = Date.now();
+      clearInterval(wsHeartbeatTimer);
+      wsHeartbeatTimer = setInterval(() => {
+        if (Date.now() - wsLastMessageAt > WS_DEAD_AFTER_MS) {
+          switchToSSE('WebSocket liefert keine Antworten mehr (Proxy leitet nichts weiter)');
+          return;
+        }
+        try {
+          ws.send(JSON.stringify({ event: 'PING' }));
+        } catch {}
+      }, WS_PING_INTERVAL_MS);
       if (activeGlobalSSE) {
         try { activeGlobalSSE.close(); } catch {}
         activeGlobalSSE = null;
@@ -1295,8 +1334,10 @@ export function connectWebSocket(): void {
     };
 
     ws.onmessage = (event) => {
+      wsLastMessageAt = Date.now();
       try {
         const msg = JSON.parse(event.data) as WSMessage;
+        if (msg.event === 'PONG') return;
         wsListeners.forEach((fn) => {
           try {
             fn(msg);
@@ -1312,6 +1353,7 @@ export function connectWebSocket(): void {
 
     ws.onclose = (evt: CloseEvent) => {
       clearTimeout(wsConnectionWatchdog);
+      clearInterval(wsHeartbeatTimer);
       activeGlobalWS = null;
 
       // In corporate environments, blocked WebSockets close abruptly without clean handshake
@@ -1368,6 +1410,8 @@ if (typeof window !== 'undefined') {
 }
 
 export function disconnectWebSocket(): void {
+  clearInterval(wsHeartbeatTimer);
+  clearTimeout(sseFirstEventWatchdog);
   manualDisconnect = true;
   clearTimeout(wsReconnectTimer);
   clearTimeout(wsConnectionWatchdog);