feat: add export endpoint with WebSocket progress

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

server/ws.py

@@ -0,0 +1,37 @@
+import asyncio
+import json
+
+from fastapi import WebSocket, WebSocketDisconnect
+
+_connections: list[WebSocket] = []
+
+
+async def connect(ws: WebSocket):
+    await ws.accept()
+    _connections.append(ws)
+    try:
+        while True:
+            await ws.receive_text()  # keep alive
+    except WebSocketDisconnect:
+        _connections.remove(ws)
+
+
+def broadcast(msg: dict):
+    """Send a message to all connected WebSocket clients.
+
+    Called from sync code (export callbacks), so we schedule the coroutine
+    on each connection's event loop.
+    """
+    data = json.dumps(msg)
+    stale = []
+    for ws in _connections:
+        try:
+            loop = asyncio.get_event_loop()
+            if loop.is_running():
+                asyncio.run_coroutine_threadsafe(ws.send_text(data), loop)
+            else:
+                loop.run_until_complete(ws.send_text(data))
+        except Exception:
+            stale.append(ws)
+    for ws in stale:
+        _connections.remove(ws)