Authenticated Server-Side Request Forgery (SSRF) via feed URL in Stringer

Stringer is a self-hosted, anti-social RSS reader. Prior to commit 75cb095, an unrestricted Server-Side Request Forgery (SSRF) vulnerability allows any authenticated user to force the Stringer server to send arbitrary HTTP/HTTPS requests to internal networks, localhost services, and cloud metadata endpoints (e.g. AWS IMDS 169.254.169.254). When self-service signup is enabled (Setting::UserSignup), even a low-privileged registered user can exploit this to scan internal services or steal cloud IAM credentials. This issue has been patched via commit 75cb095.