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

[dkdev179]

🚨 Severity: HIGH
💡 Vulnerability: API token comparison was using a variable-time string comparison (!=), making it susceptible to timing attacks.
🎯 Impact: Attackers could potentially infer the API token character by character by measuring the response time, which may lead to unauthorized access.
🔧 Fix: Replaced != with secrets.compare_digest in mcp/api.py for a constant-time comparison.
✅ Verification: Ensure the provided test suite (python -m pytest tests/) still successfully executes and valid tokens continue to authorize properly. Also recorded the finding in .jules/sentinel.md.

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PR created automatically by Jules for task 1021281418732237299 started by @dkdev179

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