Add lightweight health check endpoint for load balancers to probe

What does this MR do and why?

This adds a small, fixed HTTP endpoint that just responds {"status":"ok"} when queried. This can be probed by load balancers and other check systems to see if Crafty is alive.

Screenshots or screen recordings

$ curl https://hostname.example.com:8443/api/v2/crafty/check
{"status":"ok"}

How to set up and validate locally

Pure local check

1. install Crafty
2. curl https://hostname.example.com:8443/api/v2/crafty/check
3. Expect return string {"status":"ok"}

Load balancer config

This was tested against HAProxy with a similar backend config snippet (minorly edited from the source server):

backend be_crafty_hostname
    # testing crafty backend
    http-check connect ssl
    http-check send meth GET uri /api/v2/crafty/check
    http-check expect status 200
    http-check expect string '{"status":"ok"}'
    default-server ssl check inter 15s check-ssl verify required
    server  hostname    hostname.example.com:8443  ca-file /etc/haproxy/tls-be/hostname-crafty.crt

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Have you checked this doesn't interfere/conflict/duplicate someone elses work?
• Have you fully tested your changes?
• Have you resolved any lint issues?
• Have you assigned a reviewer?
• Have you applied correct labels?