* refac(routers): reject external URLs in profile/model image handlers
* refac(ui): centralize image URL validation in safeImageUrl helper
* refac(auths): make signout POST-only
* refac: gate external profile image redirect behind ENABLE_PROFILE_IMAGE_URL_FORWARDING
Restore the 302 redirect for external http(s) profile image URLs in
the user and model profile-image endpoints, but gate it behind a new
ENABLE_PROFILE_IMAGE_URL_FORWARDING env flag (default: True).
Existing deployments that rely on external profile image forwarding
continue to work unchanged. Operators who want to suppress the
redirect (to prevent client-side IP/UA/Referer leaks) can set the
flag to False.
