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

[google-labs-jules]

🚨 Severity: CRITICAL
💡 Vulnerability: Short-circuiting token validation by checking length before crypto.timingSafeEqual leaks the secret's length via timing differences.
🎯 Impact: An attacker could perform a timing attack to guess the token length and potentially the token itself.
🔧 Fix: Hash both secrets to a fixed length (using crypto.createHash('sha256')) before comparison to ensure true constant-time evaluation and remove the length check.
✅ Verification: bun run test:full runs successfully.

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PR created automatically by Jules for task 5495307799069338049 started by @YoungSx

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