🛡️ Sentinel: [CRITICAL] Fix DoS and timing leaks in admin authentication

[ralksta]

🚨 Severity: CRITICAL
💡 Vulnerability: Missing length validation in crypto.timingSafeEqual and unconstrained Buffer.alloc() based on user input lengths.
🎯 Impact: Attackers can cause Denial of Service (DoS) through unhandled exceptions in token validation by sending signatures of incorrect lengths. Attackers can also trigger memory exhaustion DoS via huge variable-length buffers when verifying passwords.
🔧 Fix: Added explicit buffer length checks before timingSafeEqual in token verification. Enforced a maximum string length on password input and replaced variable-length buffer allocations with fixed-length SHA-256 hashing prior to comparison in verifyAdminPassword.
✅ Verification: Tested manual invalid token lengths and huge password inputs to ensure they no longer cause unhandled exceptions or massive memory allocation. Verified changes with test suite and linting.

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PR created automatically by Jules for task 1937367240464957059 started by @ralksta

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