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/README.md

@@ -25,7 +25,7 @@ pipecat 把"管线"和"输出方式"解耦:同一条 `STT→LLM→TTS` 管线可
 ```
 ai-video-backend/
 ├── app.py                       # FastAPI 入口,挂路由 + CORS
-├── config.py                    # 读 .env,模型接口环境变量兜底
+├── settings.py                  # 读 .env,仅存数据库/CORS/TURN 等运行设置
 ├── models.py                    # AssistantConfig(对齐前端 AssistantForm)
 ├── routes/                      # 一个文件一组端点(对齐 dograh routes/)
 │   ├── health.py
@@ -96,7 +96,7 @@ uv pip install fastapi "uvicorn[standard]" sqlalchemy asyncpg greenlet python-do
 # 阶段 B:做语音时再装全量(含 pipecat,需 3.10+)
 # uv pip install -r requirements.txt
 
-cp .env.example .env          # CRUD 阶段只需 DATABASE_URL;语音再填模型 key
+cp .env.example .env          # 只放数据库/CORS/TURN;模型 key 在模型资源里维护
 # 起 Postgres:在 ai-video/ 下 docker compose up -d postgres
 cd ..
 make db-migrate               # 首次或表结构变更后执行 Alembic 迁移