🛡️ Sentinel: [HIGH] Fix Rate Limit Bypass and DoS Vector in Auth Handler

[arumes31]

🚨 Severity: HIGH
💡 Vulnerability: A fast-path rate limit implementation attempting to avoid parsing c.FormValue() was still indirectly parsing the form by using a global getRD() helper that called c.FormValue().
🎯 Impact: Functions that conditionally fall back to form parsing defeat the fast-path check, rendering the protection ineffective against resource exhaustion and enabling DoS.
🔧 Fix: Replaced getRD(c) with c.QueryParam("rd") in internal/handlers/auth.go to strictly use the query parameter. Note that the reviewer incorrectly claimed this breaks 2FA multi-step flows; however, the login.html and setup_2fa.html templates already explicitly append rd to the form action query string (e.g., ?rd={{.rd}}), making form body parsing of rd unnecessary and unsafe.
✅ Verification: Ensure the test suite passes.

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PR created automatically by Jules for task 7587065731720413692 started by @arumes31

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