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

[dkdev179]

🚨 Severity: CRITICAL
💡 Vulnerability: The API token validation used the standard != operator, which short-circuits and is susceptible to timing attacks that could allow attackers to incrementally guess the API_TOKEN.
🎯 Impact: An attacker could potentially bypass authentication by deducing the token based on response times.
🔧 Fix: Replaced the vulnerable string comparison with secrets.compare_digest(), ensuring constant-time token comparison that resists timing attacks.
✅ Verification: Verify that unit tests pass and code formats properly. The patch modifies mcp/api.py.

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PR created automatically by Jules for task 13326497848829876451 started by @dkdev179

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