Remove env var overrides, config.yaml is single source of truth

- Stripped _ENV_MAP and _apply_env_overrides from configs/load.py
- Cleaned up unused imports in video_receiver.py
- Restored MediaPipe env suppressors in app/main.py
- Removed .env.example (replaced by config.yaml)

app/webrtc/video_receiver.py

@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import asyncio
-import os
 
 import cv2
 from aiortc.mediastreams import MediaStreamError