Refactor backend to support interface-definition driven model resources

- Introduce a new model structure for managing interface definitions and model resources, enhancing the backend's capability to handle various service integrations.
- Update the Makefile to reflect changes in database seeding and resource management commands.
- Remove the deprecated credentials management routes and replace them with a unified model registry API.
- Modify existing routes and schemas to align with the new model structure, ensuring seamless integration with the frontend.
- Enhance database seeding scripts to populate new model resources and their configurations.
- Update README documentation to reflect the new architecture and usage instructions for model resources and interface definitions.

backend/config.py

@@ -1,4 +1,4 @@
-"""集中读取环境变量。所有 provider 的接入点都在这里,改栈只改 .env。"""
+"""集中读取环境变量。所有模型接口的环境变量兜底都在这里。"""
 
 import os