feat: support realtime websocket resubscribe

[doskoi]

Summary

• allow /ws/realtime clients to update subscriptions within the same WebSocket connection
• support action=set, action=subscribe, and action=unsubscribe messages
• clean up replaced subscriptions on period changes and disconnect
• add tests using fake xtdata and WebSocket objects

Why

The endpoint currently handles the initial subscription request, then ignores later client messages until disconnect. Long-lived realtime clients often need to add/remove symbols or replace the watched list without reconnecting.

Protocol

Existing clients remain compatible because omitted action defaults to set.

{"stocks": ["000001.SZ", "600000.SH"], "period": "tick"}
{"action": "subscribe", "stocks": ["300750.SZ"]}
{"action": "unsubscribe", "stocks": ["600000.SH"]}
{"action": "set", "stocks": ["000001.SZ"], "period": "1m"}

Tests

• uv run --with pytest --with fastapi --with numpy --with pandas python -m pytest tests/test_realtime_resubscribe.py -q