Sentinel: [CRITICAL] Fix Arbitrary Command Execution in Rhai Engine

[JustinWoodring]

🛡️ Sentinel: [CRITICAL] Fix Arbitrary Command Execution in Rhai Engine

🚨 Severity: CRITICAL
💡 Vulnerability: The shell_execute function in slatron-server allowed execution of arbitrary shell commands.
🎯 Impact: An attacker with script editing privileges could execute arbitrary code on the server, potentially compromising the entire system.
🔧 Fix: Implemented an explicit allowlist for shell_execute in slatron-server/src/rhai_engine/mod.rs. Only yt-dlp, ffmpeg, and ffprobe are allowed.
✅ Verification: Added test_shell_execute_allowlist in slatron-server/src/rhai_engine/security_test.rs which verifies that disallowed commands (e.g., ls) are rejected with a security error, while allowed commands pass the check. Verified by running cargo test in slatron-server.

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PR created automatically by Jules for task 5448247204337572309 started by @JustinWoodring

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