🛡️ Sentinel: [CRITICAL/HIGH] Fix path traversal vulnerability in snapshot names

[bitcoiner-dev]

🚨 Severity: CRITICAL
💡 Vulnerability: The snapshot save and snapshot restore commands accepted unvalidated user input for the snapshot name, which was directly passed to Path::join(). This allowed a path traversal attack (e.g., ../etc/passwd) to read or overwrite arbitrary files on the user's system under the context of the running application.
🎯 Impact: An attacker could overwrite critical system files or expose sensitive data by providing malicious path traversals as snapshot names.
🔧 Fix: Implemented an explicit allowlist validation crate::utils::validate_file_name that restricts input to alphanumeric characters, dashes, and underscores. This validation is now enforced for all snapshot names prior to joining with the snapshot directory path.
✅ Verification: cargo test confirms the path validation rejects any input containing . or /, ensuring safe path execution.

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PR created automatically by Jules for task 6644483708482754008 started by @bitcoiner-dev

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