Add CORS for staging domain and update docker-compose

- Add https://staging.autonetsellcar.com to CORS allowed origins
- Update docker-compose.staging.yml to use .env file and correct paths

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/main.py

@@ -171,6 +171,8 @@ app.add_middleware(
         "http://192.168.0.202:8001",  # Staging backend
         "https://autonetsellcar.com",  # Production domain
         "http://autonetsellcar.com",   # Production (HTTP redirect)
+        "https://staging.autonetsellcar.com",  # Staging domain
+        "http://staging.autonetsellcar.com",   # Staging (HTTP redirect)
     ],
     allow_credentials=True,
     allow_methods=["*"],