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/settings.py

@@ -0,0 +1,39 @@
+"""Runtime settings for backend infrastructure.
+
+Model provider credentials live in ``model_resources`` and are resolved per
+assistant. Keep this module limited to process-level settings such as database,
+CORS, server bind address, and WebRTC ICE/TURN configuration.
+"""
+
+import os
+
+from dotenv import load_dotenv
+
+load_dotenv()
+
+
+def _split(value: str) -> list[str]:
+    return [item.strip() for item in value.split(",") if item.strip()]
+
+
+# ---- Database ----
+DATABASE_URL = os.getenv(
+    "DATABASE_URL",
+    "postgresql+asyncpg://postgres:postgres@localhost:5432/postgres",
+)
+
+# ---- Service ----
+HOST = os.getenv("HOST", "0.0.0.0")
+PORT = int(os.getenv("PORT", "8000"))
+CORS_ORIGINS = _split(
+    os.getenv("CORS_ORIGINS", "http://localhost:3000,http://127.0.0.1:3000")
+)
+
+# ---- WebRTC TURN ----
+# TURN_URLS example:
+# turn:182.92.86.220:3478?transport=udp,turn:182.92.86.220:3478?transport=tcp
+TURN_URLS = _split(os.getenv("TURN_URLS", ""))
+TURN_SECRET = os.getenv("TURN_SECRET", "")
+TURN_USERNAME = os.getenv("TURN_USERNAME", "")
+TURN_PASSWORD = os.getenv("TURN_PASSWORD", "")
+TURN_CREDENTIAL_TTL = int(os.getenv("TURN_CREDENTIAL_TTL", "86400"))