chore: import upstream snapshot with attribution

docs/md/how_to/python/virtual_server/clickhouse.md

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+# ClickHouse Virtual Server
+
+Perspective provides a built-in virtual server for
+[ClickHouse](https://clickhouse.com/), allowing `<perspective-viewer>` clients
+to query a ClickHouse server over WebSocket.
+
+For browser-only usage, see the
+[JavaScript ClickHouse guide](../../javascript/virtual_server/clickhouse.md).
+
+## Installation
+
+```bash
+pip install perspective-python clickhouse-connect
+```
+
+## Usage
+
+Create a server that exposes ClickHouse tables to browser clients:
+
+```python
+import clickhouse_connect
+import tornado.web
+import tornado.ioloop
+from perspective import ClickhouseVirtualServer
+from perspective.handlers.tornado import PerspectiveTornadoHandler
+
+# Connect to ClickHouse
+client = clickhouse_connect.get_client(host="localhost")
+
+# Create virtual server backed by ClickHouse
+server = ClickhouseVirtualServer(client)
+
+# Serve over WebSocket
+app = tornado.web.Application([
+    (r"/websocket", PerspectiveTornadoHandler, {"perspective_server": server}),
+])
+
+app.listen(8080)
+tornado.ioloop.IOLoop.current().start()
+```
+
+Connect from the browser:
+
+```javascript
+const websocket = await perspective.websocket("ws://localhost:8080/websocket");
+const table = await websocket.open_table("my_table");
+document.getElementById("viewer").load(table);
+```
+
+## Examples
+
+- [Python ClickHouse example](https://github.com/perspective-dev/perspective/tree/master/examples/python-clickhouse-virtual)