🛡️ Sentinel: [HIGH] Fix timing attack vulnerability in token validation

[google-labs-jules]

🚨 Severity: HIGH
💡 Vulnerability: Checking token lengths before calling timingSafeEqual creates a short-circuit fast path that leaks the secret's length via timing differences.
🎯 Impact: An attacker could potentially deduce the length of valid authentication tokens and theoretically execute a timing attack.
🔧 Fix: Hash both the input token and valid tokens to a fixed length using SHA-256 before comparison with timingSafeEqual.
✅ Verification: Ran bun test tests/unit/cli/commands/serve.test.ts and the full test suite.

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PR created automatically by Jules for task 10992808094530778128 started by @YoungSx

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