🛡️ Sentinel: [CRITICAL] Fix path traversal in API file retrieval

[thirdeyenation]

🚨 Severity: CRITICAL
💡 Vulnerability: Path traversal vulnerability in api_files_get allowing arbitrary file read outside the base directory using ../ or absolute paths.
🎯 Impact: An attacker could read sensitive files on the server (like /etc/passwd or configuration files) by crafting malicious paths.
🔧 Fix: Added a security check using files.is_in_base_dir to ensure resolved paths are strictly contained within the intended base directory before reading them.
✅ Verification: Tested via a Python script simulating malicious payload requests, confirming that outside paths are rejected and valid paths still work.

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PR created automatically by Jules for task 1934856550127019594 started by @thirdeyenation

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