🛡️ Sentinel: [security improvement] Secure snapshot directory creation

[bitcoiner-dev]

🚨 Severity: MEDIUM
💡 Vulnerability: Snapshot directory created with permissive permissions via std::fs::create_dir_all.
🎯 Impact: On Unix-like systems, if a user has a permissive umask (e.g. 022), creating a directory with std::fs::create_dir_all results in standard permissions (e.g. 755). This would make the snapshot backups readable by any other user on the system.
🔧 Fix: Replaced std::fs::create_dir_all with the repository's secure wrapper crate::paths::create_secure_dir_all to explicitly enforce 0o700 restricted permissions.
✅ Verification: Ran unit tests. The change modifies the permission attributes of directories explicitly. Verify by checking created snapshot directories with ls -ld.

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PR created automatically by Jules for task 6420376848358620843 started by @bitcoiner-dev

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