🛡️ Sentinel: [HIGH] Fix DoS vector in authentication flow

[arumes31]

🚨 Severity: HIGH
💡 Vulnerability: The getRD() function in the authentication flow checked the request body form values for the redirect parameter (rd). This required parsing the entire request body before applying basic rate limiting, exposing the application to a Denial of Service (DoS) attack where an attacker could send large POST bodies to consume server resources and bypass early rate-limit drops.
🎯 Impact: Attackers could exhaust server memory and CPU by sending a flood of large, unauthenticated POST requests, leading to application unavailability.
🔧 Fix: Replaced getRD(c) with c.QueryParam("rd") across internal/handlers/auth.go to strictly read the redirect parameter from the URL query string. The getRD helper function was deleted. This allows rate limits to drop malicious requests immediately without parsing the form body.
✅ Verification: Verified that tests pass and the code compiles successfully. The multi-step 2FA flow remains intact because the login templates already append the rd parameter to the form action query string (e.g., action="/rauthlogin?rd=...").

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PR created automatically by Jules for task 11174331091851025111 started by @arumes31

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