This commit implements a system to periodically verify the functionality
of image proxies. It adds a new environment variable
PIXIVFE_PROXY_CHECK_INTERVAL to control the check interval, defaulting
to 8 hours.
The change includes a new proxy_checker.go file with the core logic for
the proxy checker, updates to config.go to support the new feature, as
well as modifications to main.go and settings.go to integrate the
checker.
iacore
VnPower
perennial