Apply or Config.OLLAMA_MODEL safety net to assist route

The same empty-string model guard added to chat.py was missing from the
notes assist route. If default_model was stored as "" in the DB, the
assist route would pass "" to Ollama which responds with 400, surfaced
to the user as a "400" error in the assist panel.

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

src/fabledassistant/routes/notes.py

@@ -259,7 +259,7 @@ async def assist_route():
     if existing and existing.state == GenerationState.RUNNING:
         return jsonify({"error": "Assist generation already running"}), 409
 
-    model = await get_setting(uid, "default_model", Config.OLLAMA_MODEL)
+    model = await get_setting(uid, "default_model", Config.OLLAMA_MODEL) or Config.OLLAMA_MODEL
     messages = build_assist_messages(body, target_section, instruction)
 
     buf = create_assist_buffer(uid)