feat(sse): sync temps réel multi-appareils via Server-Sent Events v0.5.8

- Broadcaster asyncio.Queue avec keepalive 25s (prévient timeout proxy)
- Endpoint GET /api/events/stream (StreamingResponse text/event-stream)
- Broadcast notes_changed / todos_changed / shopping_changed sur toutes mutations
- Hook useServerEvents: EventSource avec reconnexion automatique (3s)
- Pages Notes, Todos, Shopping abonnées aux événements SSE
- nginx: location SSE dédiée (proxy_buffering off, timeout 24h)

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

backend/app/core/broadcaster.py

@@ -0,0 +1,36 @@
+import asyncio
+import json
+from typing import AsyncGenerator
+
+
+class EventBroadcaster:
+    def __init__(self):
+        self._queues: set[asyncio.Queue] = set()
+
+    async def subscribe(self) -> AsyncGenerator[str, None]:
+        queue: asyncio.Queue[str] = asyncio.Queue(maxsize=32)
+        self._queues.add(queue)
+        try:
+            while True:
+                try:
+                    msg = await asyncio.wait_for(queue.get(), timeout=25)
+                    yield msg
+                except asyncio.TimeoutError:
+                    yield ": keepalive\n\n"
+        except asyncio.CancelledError:
+            pass
+        finally:
+            self._queues.discard(queue)
+
+    def broadcast(self, event_type: str, data: dict | None = None) -> None:
+        if not self._queues:
+            return
+        msg = f"event: {event_type}\ndata: {json.dumps(data or {})}\n\n"
+        for queue in list(self._queues):
+            try:
+                queue.put_nowait(msg)
+            except asyncio.QueueFull:
+                pass
+
+
+broadcaster = EventBroadcaster()