Refactor backend configuration management and update environment settings

- Replace the `config.py` module with a new `settings.py` to streamline environment variable management, focusing on database, CORS, and TURN settings.
- Update references throughout the backend codebase to use the new `settings` module instead of the deprecated `config`.
- Modify the `.env.example` file to reflect the new configuration approach, indicating that model provider credentials should be maintained separately.
- Enhance the `AssistantConfig` model to clarify the source of runtime connection information, ensuring it is injected from model resources rather than relying on defaults from the environment.
- Introduce new user scripts for audio and video management in the Tampermonkey environment, enhancing WebRTC capabilities.

backend/models.py

@@ -70,7 +70,7 @@ class AssistantConfig(BaseModel):
     fastgpt_app_id: str = ""
 
     # ---- 运行时连接信息(服务端注入,不来自浏览器) ----
-    # 为空时,service_factory 会回退到 config.py 的 .env 默认值。
+    # 由模型资源注入;调试 inline_config 也必须显式提供。
     llm_api_key: str = ""
     llm_base_url: str = ""
     stt_api_key: str = ""