feat(chat): add tool_calls param to add_message for synthetic messages

Needed by the Discuss endpoint to persist synthetic read_article
tool exchanges before triggering generation.

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

src/fabledassistant/services/chat.py

@@ -186,6 +186,7 @@ async def add_message(
     content: str,
     context_note_id: int | None = None,
     status: str | None = None,
+    tool_calls: list | None = None,
 ) -> Message:
     async with async_session() as session:
         kwargs: dict = dict(
@@ -196,6 +197,8 @@ async def add_message(
         )
         if status is not None:
             kwargs["status"] = status
+        if tool_calls is not None:
+            kwargs["tool_calls"] = tool_calls
         msg = Message(**kwargs)
         session.add(msg)
         # Touch conversation updated_at