feat(briefing): extend post_message() to accept optional metadata dict

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

src/fabledassistant/services/briefing_conversations.py

@@ -41,7 +41,12 @@ async def get_or_create_today_conversation(user_id: int, model: str) -> Conversa
         return conv
 
 
-async def post_message(conversation_id: int, role: str, content: str) -> Message:
+async def post_message(
+    conversation_id: int,
+    role: str,
+    content: str,
+    metadata: dict | None = None,
+) -> Message:
     """Append a message to a briefing conversation."""
     async with async_session() as session:
         msg = Message(
@@ -49,6 +54,7 @@ async def post_message(conversation_id: int, role: str, content: str) -> Message
             role=role,
             content=content,
             status="complete",
+            msg_metadata=metadata,
         )
         session.add(msg)
         # Bump conversation updated_at