Update environment variable loading to allow overrides

- Modified load_dotenv() calls in multiple files to include override=True, ensuring that .env variables can replace existing environment variables.
- Updated ota_storage.py to use default values for OTA upload directories, improving clarity and maintainability.
- Enhanced get_ota_upload_dir() function to read environment variables dynamically, ensuring accurate directory paths based on the current environment.

backend/security.py

@@ -8,7 +8,7 @@ from dotenv import load_dotenv
 from fastapi import HTTPException, Security
 from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
 
-load_dotenv()
+load_dotenv(override=True)
 
 logger = logging.getLogger(__name__)