feat(config): add fallback model support for AI configurations

configs/config.example.yaml

@@ -42,6 +42,7 @@ ai:
     dimensions: 1536
   metadata:
     model: "gpt-4o-mini"
+    fallback_model: ""
     temperature: 0.1
   litellm:
     base_url: "http://localhost:4000/v1"
@@ -50,6 +51,7 @@ ai:
     request_headers: {}
     embedding_model: "openrouter/openai/text-embedding-3-small"
     metadata_model: "gpt-4o-mini"
+    fallback_metadata_model: ""
   ollama:
     base_url: "http://localhost:11434/v1"
     api_key: "ollama"