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/core/logging.py

@@ -12,7 +12,7 @@ def setup_logging() -> None:
     log_dir.mkdir(parents=True, exist_ok=True)
 
     logger.add(
-        log_dir / "posefit-server_{time:YYYY-MM-DD}.log",
+        log_dir / "{time:YYYY-MM-DD}.log",
         rotation=config.logging.rotation,
         retention=config.logging.retention,
         enqueue=True,