🛡️ Sentinel: [HIGH] Fix SSRF bypass via unvalidated HTTP redirects in proxy_stream

[d3mocide]

🚨 Severity: HIGH
💡 Vulnerability: The /radio/proxy/{stream_id} endpoint instantiated an httpx.AsyncClient with follow_redirects=True. Although the initial URL is checked with validate_safe_url, if an external malicious server returns an HTTP 302 redirect pointing to an internal IP (like 127.0.0.1 or 169.254.169.254), httpx automatically follows it without triggering another SSRF check, bypassing the security control.
🎯 Impact: An attacker could bypass SSRF protections and proxy requests to internal backend services by serving redirects.
🔧 Fix: Added an event_hooks={'request': [_validate_request_url]} callback to the httpx.AsyncClient which intercepts and runs validate_safe_url on every outgoing request during the client's lifecycle, fully blocking redirects to internal IPs.
✅ Verification: Ran python3 -m unittest discover backend/tests/ to verify tests pass and ran manual testing to confirm redirects to 127.0.0.1 are successfully blocked by httpx.

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PR created automatically by Jules for task 11510504226126764344 started by @d3mocide

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