🛡️ Sentinel: [CRITICAL] Fix PDF compilation RCE and DoS vulnerability

[anchapin]

🚨 Severity: CRITICAL
💡 Vulnerability: CoverLetterGenerator and PDFConverter used subprocess.Popen to call pdflatex and pandoc without specifying the -no-shell-escape flags, opening the possibility for Remote Code Execution (RCE) if a malicious user injects LaTeX shell escape commands. In addition, no timeout argument was passed to process.communicate(), introducing a Denial of Service (DoS) vulnerability where infinite compilation loops could hang the server.
🎯 Impact: Attackers could execute arbitrary shell commands on the server running the application or crash the server by providing input that causes infinite compilation loops.
🔧 Fix: Enforce the -no-shell-escape flag for pdflatex (directly) and pandoc (via --pdf-engine-opt=-no-shell-escape). Implement subprocess timeouts (timeout=30) with explicit process cleanup (catching subprocess.TimeoutExpired, executing process.kill(), and calling process.communicate() a second time) to prevent zombie processes and double-free issues.
✅ Verification: The test suite has been run using pytest and all tests pass. Tested specifically the test_pdf_security.py tests.

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PR created automatically by Jules for task 11253958845217151392 started by @anchapin

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