🛡️ Sentinel: [CRITICAL] Fix hardcoded 2FA bypass backdoor

[secrooq]

🚨 Severity: CRITICAL
💡 Vulnerability: A hardcoded bypass secret ("stealth_dev_bypass_root_2026") was used as a backdoor for 2FA verification in src/auth.ts.
🎯 Impact: This allowed potential unauthorized access if the secret was known.
🔧 Fix: Updated the check to require the environment variable ADMIN_BYPASS_SECRET. Default expected secret is now null and it will only bypass 2FA if the environment variable is explicitly set and matches the entered code.
✅ Verification: Ran unit tests and end-to-end playwright tests successfully.

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PR created automatically by Jules for task 13236577053826937398 started by @secrooq

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