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

[dkdev179]

🚨 Severity: CRITICAL
💡 Vulnerability: The API token verification used standard string comparison (!=), which fails fast and leaks time information, allowing attackers to perform a timing attack to guess the token length and content.
🎯 Impact: An attacker could potentially brute-force or deduce the API token by analyzing the response times of the API requests, gaining unauthorized access to the application endpoints.
🔧 Fix: Used secrets.compare_digest to perform a constant-time comparison on the API token and the user-provided token, preventing the timing information leakage.
✅ Verification: Ran python -m pytest tests/test_api.py and the tests passed successfully, verifying that the authentication logic remains functionally identical while becoming secure.

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PR created automatically by Jules for task 4038169625846520875 started by @dkdev179

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