🛡️ Sentinel: [CRITICAL] Fix Timing Attack in Scheduler Secret Validation

[ivanleekk]

🚨 Severity: CRITICAL
💡 Vulnerability: Timing attack vulnerability in the internal task scheduler due to standard string equality comparison (!=) for a secret header (x_scheduler_secret).
🎯 Impact: Attackers could potentially deduce the scheduler secret character by character by measuring the response times of the server.
🔧 Fix: Replaced standard string comparison with constant-time comparison secrets.compare_digest(). Added a None check before comparison to prevent TypeError exceptions which could cause unintentional 500 errors.
✅ Verification: Ran ruff check src/routers/internal.py and it passed. Re-ran backend tests without introducing any new failures.

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PR created automatically by Jules for task 13574772256673558941 started by @ivanleekk

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