🛡️ Sentinel: Fix hardcoded JWT secret fallback

[Pmaster-dev]

🚨 Severity: HIGH
💡 Vulnerability: Fallback to static 'jwt-secret' when JWT_SECRET_KEY environment variable is not set.
🎯 Impact: Attackers could forge or decode JWT tokens if JWT_SECRET_KEY environment variable is missing.
🔧 Fix: Require JWT_SECRET_KEY to be set in environment and raise ValueError if missing or empty in JWTUtils.
✅ Verification: Added unit tests in tests/test_auth_utils.py ensuring missing JWT_SECRET_KEY raises ValueError and token operations succeed when set.

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PR created automatically by Jules for task 4755741623692660761 started by @Pmaster-dev

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