🛡️ Sentinel: [CRITICAL] Fix authentication bypass vulnerability

[secrooq]

🚨 Severity: CRITICAL
💡 Vulnerability: A hardcoded 2FA bypass secret ("stealth_dev_bypass_root_2026") was present in src/auth.ts, allowing an attacker who knows this secret to bypass the Two-Factor Authentication requirement if the ADMIN_BYPASS_SECRET environment variable is not explicitly configured on the server.
🎯 Impact: Attackers could gain unauthorized access by bypassing 2FA mechanisms entirely, leading to full account compromise.
🔧 Fix: Removed the hardcoded secret and replaced it with a null fallback. The 2FA bypass check now inherently fails if ADMIN_BYPASS_SECRET is unset, following the fail-secure principle.
✅ Verification: Tested via unit tests, build generation, and e2e playwright verification tests which all passed. Also inspected src/auth.ts to confirm there is no fallback string for expectedSecret.

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PR created automatically by Jules for task 18079393030119125724 started by @secrooq

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