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/database.py

@@ -4,8 +4,8 @@ from sqlalchemy.orm import sessionmaker
 import os
 from dotenv import load_dotenv
 
-# Load environment variables
-load_dotenv()
+# Load environment variables（override=True：.env 覆盖已有环境变量，避免系统里 DEBUG=true 导致与 .env 中 DEBUG=False 冲突）
+load_dotenv(override=True)
 
 # Database configuration
 DATABASE_HOST = os.getenv("DATABASE_HOST", "localhost")