🛡️ Sentinel: [HIGH] Fix path traversal in file uploads

[adihex]

🚨 Severity: HIGH
💡 Vulnerability: A path traversal vulnerability existed in the audio upload handler (/transcribe) because path.basename does not strip Windows-style path separators (\) when running in a POSIX environment. An attacker could bypass filename extraction by supplying a filename like ..\..\..\etc\passwd, which would be processed as a literal string by path.basename and resolved as an absolute path by path.join, leading to arbitrary file write.
🎯 Impact: Arbitrary file write to the server's filesystem, which could lead to remote code execution (RCE) or system compromise.
🔧 Fix: Replaced path.basename(file.name) with a strict regex substitution file.name.replace(/[^a-zA-Z0-9.-]/g, "_"). This sanitizes the filename to only contain safe alphanumeric characters, dots, and hyphens, completely removing any risk of directory traversal characters.
✅ Verification: Upload an audio file with a filename containing ..\..\ to the /transcribe endpoint and observe that the saved file name uses underscores instead of backslashes, effectively rendering the traversal string harmless. Ensure tests run successfully (pnpm test).

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PR created automatically by Jules for task 12258315872887433358 started by @adihex

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