Add basic healthcheck endpoint

[ipmb]

Helpful for hosting environments which require/allow a healthcheck on the app. Some examples:

• https://docs.aws.amazon.com/apprunner/latest/dg/manage-configure-healthcheck.html
• https://cloud.google.com/run/docs/configuring/healthchecks
• https://render.com/docs/deploys#health-checks

[ipmb]

Might be worthwhile to wait for this to land rails/rails#46936

[andrewculver]

@ipmb Hey Peter, thanks for putting rails/rails#46936 on my radar. We'll wait for it!