Add planner previews and streaming

tests/test_streamer.py

@@ -0,0 +1,41 @@
+import asyncio
+
+from server.streamer import ScreenshotStreamer
+
+
+class DummyWebSocket:
+    def __init__(self):
+        self.sent = []
+        self.accepted = False
+
+    async def accept(self) -> None:
+        self.accepted = True
+
+    async def send_json(self, payload):
+        self.sent.append(payload)
+
+
+def test_streamer_broadcasts_to_grid():
+    streamer = ScreenshotStreamer()
+    socket = DummyWebSocket()
+
+    async def scenario():
+        key = await streamer.connect(socket, "grid-123")
+        await streamer.broadcast("grid-123", {"frame": 1})
+        streamer.disconnect(socket, key)
+
+    asyncio.run(scenario())
+    assert socket.sent == [{"frame": 1}]
+
+
+def test_streamer_wildcard_listener_receives_updates():
+    streamer = ScreenshotStreamer()
+    socket = DummyWebSocket()
+
+    async def scenario():
+        key = await streamer.connect(socket, None)
+        await streamer.broadcast("grid-456", {"frame": 2})
+        streamer.disconnect(socket, key)
+
+    asyncio.run(scenario())
+    assert socket.sent == [{"frame": 2}]