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>

Doku/reverse-proxy.md

@@ -193,6 +193,10 @@ VirtualHost-Konfiguration (`/etc/apache2/sites-available/mindsift.conf`):
 
 ---
 
+## Wenn der Proxy nicht angepasst werden kann
+
+MindSift funktioniert auch hinter Proxys, die WebSocket blocken, Streams puffern oder nur kurze Anfragen erlauben. Die Notiz-Synchronisation wählt automatisch den besten verfügbaren Weg (WebSocket → SSE → Long-Polling → Abfrage alle 3 s); der Editor zeigt ihn an. Long-Polling-Anfragen dauern höchstens 25 s und bleiben damit unter üblichen Proxy-Grenzen. Details: [api/websocket.md](api/websocket.md).
+
 ## 🔍 Diagnose & Verifikation
 
 Mit den folgenden Befehlen lässt sich die Erreichbarkeit und Streaming-Funktion prüfen: