support API Gateway instead of CloudFront (simplified configuration)

[tmokmss]

It is often reasonable to remove CloudFront and use API Gateway instead to simplify the infrastructure. Importantly, without CloudFront, we can remove the ugly workaround of circular dependencies when using cloudfront domain (see #47).

The drawbacks of using API Gateway are the following:

1. response streaming is not usable at all
2. 30 second timeout for response (default quota)
3. No cache for static files

In the simple and small systems like prototypes, it often works well though.

[tmokmss]

This update should allow more functional integration with API Gateway REST APIs.
https://aws.amazon.com/about-aws/whats-new/2025/11/api-gateway-response-streaming-rest-apis/