🛡️ Sentinel: [CRITICAL] Fix event loop blocking in SSRF validation

[d3mocide]

🚨 Severity: CRITICAL
💡 Vulnerability: The _validate_request_url event hook used to validate URLs in httpx.AsyncClient with follow_redirects=True in backend/routers/radio.py called validate_safe_url synchronously. Because validate_safe_url performs blocking DNS resolution (socket.getaddrinfo), executing it in an async context blocked the backend's main event loop, creating a Denial of Service (DoS) vulnerability.
🎯 Impact: Attackers could cause a severe latency and DoS under load.
🔧 Fix: Modified _validate_request_url in backend/routers/radio.py to run validate_safe_url within a thread using await asyncio.get_running_loop().run_in_executor(None, validate_safe_url, str(request.url)) to avoid blocking the event loop.
✅ Verification: Tested via unit tests for backend python3 -m unittest discover backend/tests. Added security learning entry to .jules/sentinel.md.

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PR created automatically by Jules for task 5840742459428934283 started by @d3mocide

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