feat(api_hooks): add WebSocketMessage + JsonValue type (t5_1-t5_8)

Phase 5 of any_type_componentization_20260621. Promotes the WebSocket
broadcast signature in src/api_hooks.py from (channel, payload: dict) to
a typed WebSocketMessage dataclass (16 Any sites):

NEW dataclass (inline in src/api_hooks.py):
- WebSocketMessage (frozen=True): channel: str, payload: JsonValue

MODIFIED:
- _serialize_for_api(obj: Any) -> JsonValue (typed return)
- broadcast(channel: str, payload: dict[str, Any]) -> broadcast(message: WebSocketMessage)
- _get_app_attr / _set_app_attr signatures UNCHANGED (Pattern 4 preserved)

NEW tests/test_api_hooks_dataclasses.py (12 tests, all pass):
- test_websocket_message_construction
- test_websocket_message_with_list_payload
- test_websocket_message_with_nested_payload
- test_websocket_message_is_frozen
- test_websocket_message_to_json
- test_serialize_for_api_returns_dict_for_to_dict_object
- test_serialize_for_api_handles_nested_lists
- test_serialize_for_api_handles_purepath
- test_serialize_for_api_passthrough_for_primitives
- test_serialize_for_api_handles_mixed_nesting
- test_get_app_attr_signature_preserved (Pattern 4 invariant)
- test_set_app_attr_signature_preserved (Pattern 4 invariant)

MODIFIED tests/test_websocket_server.py:
- Updated broadcast() call site to use WebSocketMessage(channel=..., payload=...)
- Added WebSocketMessage import

Verified:
  uv run pytest tests/test_api_hooks_dataclasses.py tests/test_api_hooks_warmup.py tests/test_websocket_server.py --timeout=30
    23 passed in 5.03s (12 new + 10 existing + 1 websocket)

tests/test_websocket_server.py

@@ -2,7 +2,7 @@ import pytest
 import asyncio
 import json
 import websockets
-from src.api_hooks import WebSocketServer
+from src.api_hooks import WebSocketMessage, WebSocketServer
 
 @pytest.mark.asyncio
 async def test_websocket_subscription_and_broadcast():
@@ -32,7 +32,7 @@ async def test_websocket_subscription_and_broadcast():
    
    # Broadcast an event from the server
    event_payload = {"event": "test_event", "data": "hello"}
-   server.broadcast("events", event_payload)
+   server.broadcast(WebSocketMessage(channel="events", payload=event_payload))
    
    # Receive the broadcast
    broadcast_response = await websocket.recv()