🛡️ Sentinel: [HIGH] Fix path traversal vulnerability in audio upload

[adihex]

🚨 Severity: HIGH
💡 Vulnerability: The /transcribe endpoint used path.basename on file.name to construct a temporary filename for audio files. In a POSIX environment, path.basename does not strip shell metacharacters or Windows-style directory separators (\), posing a risk of command injection or path traversal if that temporary file were passed into subshells.
🎯 Impact: Attackers could inject arbitrary commands when the server processes the temp file via whisper.cpp or could potentially traverse the file system if the constructed path is used elsewhere.
🔧 Fix: Sanitize file.name by replacing all non-alphanumeric characters (except dots and hyphens) with underscores (_) before appending it to the base temp path.
✅ Verification: Ran unit and e2e test suites to ensure files still upload successfully and the name parses cleanly. Tests passing.

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PR created automatically by Jules for task 1312105445582802999 started by @adihex

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