🛡️ Sentinel: [CRITICAL] Fix command injection vulnerability

[dttdrv]

🚨 Severity: CRITICAL
💡 Vulnerability: The Rust compile worker directly passed user-controlled req.mainFile to latexmk. An attacker could pass a filename starting with - (e.g., -shell-escape) to inject arguments during array-based argument passing (spawn).
🎯 Impact: Attackers could execute arbitrary commands by escaping into the shell via latexmk flags.
🔧 Fix: Updated backend/src/services/compileQueue.ts to explicitly reject any mainFile that begins with a hyphen (-) before enqueuing the job, mitigating the option injection.
✅ Verification: Tested against the vitest test suite (npx vitest run) to ensure valid configurations still work as intended and that no side effects were introduced.

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PR created automatically by Jules for task 4108359020817757289 started by @dttdrv

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