🛡️ Sentinel: [MEDIUM] Replace insecure Math.random for temp files

[Kaos599]

🚨 Severity: MEDIUM
💡 Vulnerability: Predictable temporary file paths generated using Math.random().
🎯 Impact: Math.random() is not a cryptographically secure pseudo-random number generator (CSPRNG). Using it to generate temporary file names can lead to predictable file paths, making the application vulnerable to race conditions, local file inclusion, or symlink attacks if files are written to shared directories.
🔧 Fix: Replaced Math.random().toString(16/36) with Node's native, secure crypto.randomBytes(8).toString('hex') in src/fs-utils.ts and src/backup/archive.ts.
✅ Verification: Ran bun x tsc --noEmit, bun x eslint ., and pnpm test to ensure changes compile correctly and tests pass without regressions.

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PR created automatically by Jules for task 28965670466011516 started by @Kaos599

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