Fix chat model always using conv.model instead of user setting

conv.model was stored at conversation creation time and short-circuited
the get_setting() call, so changing default_model in Settings had no
effect on existing conversations. Now the user's default_model setting
is always the source of truth for generation and summarization.

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

src/fabledassistant/routes/chat.py

@@ -136,7 +136,7 @@ async def send_message_route(conv_id: int):
         if msg.role != "system":
             history.append({"role": msg.role, "content": msg.content})
 
-    model = conv.model or await get_setting(uid, "default_model", Config.OLLAMA_MODEL) or Config.OLLAMA_MODEL
+    model = await get_setting(uid, "default_model", Config.OLLAMA_MODEL) or Config.OLLAMA_MODEL
 
     # Launch background generation task (context building happens inside the task)
     asyncio.create_task(run_generation(
@@ -259,7 +259,7 @@ async def summarize_conversation_route(conv_id: int):
     conv = await get_conversation(uid, conv_id)
     if conv is None:
         return jsonify({"error": "Conversation not found"}), 404
-    model = conv.model or await get_setting(uid, "default_model", Config.OLLAMA_MODEL) or Config.OLLAMA_MODEL
+    model = await get_setting(uid, "default_model", Config.OLLAMA_MODEL) or Config.OLLAMA_MODEL
     try:
         note = await summarize_conversation_as_note(uid, conv_id, model)
         return jsonify(note), 201