🛡️ Sentinel: [HIGH] Remove hardcoded JWT secret fallback

[Pmaster-dev]

🚨 Severity: HIGH
💡 Vulnerability: Hardcoded secret fallback ('jwt-secret') in JWT token creation and verification when JWT_SECRET_KEY is not configured.
🎯 Impact: Attackers could forge valid JWT tokens for any user identity if JWT_SECRET_KEY is omitted from environment settings.
🔧 Fix: Explicitly check for JWT_SECRET_KEY in JWTUtils. Prevent token generation (raise RuntimeError) and token decoding (return None) when JWT_SECRET_KEY is missing.
✅ Verification: Created unit tests in tests/test_auth_utils.py verifying behavior when JWT_SECRET_KEY is missing vs. configured, and verified full test suite passes.

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PR created automatically by Jules for task 5828157727268265771 started by @Pmaster-dev

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