🛡️ Sentinel: [CRITICAL] Fix insecure session file persistence

[kevinaboos]

🚨 Severity: HIGH
💡 Vulnerability: The save_session and save_latest_user_id functions in src/persistence/matrix_state.rs were using tokio::fs::write, which writes files with default permissions (potentially world-readable depending on umask) and is not atomic. The session file contains sensitive information, including the database passphrase.
🎯 Impact: Local attackers with access to the filesystem could read the session file and extract the database passphrase or hijack the user session.
🔧 Fix: Implemented write_file_securely in src/persistence/utils.rs which:
1. Writes to a temporary file with 0o600 permissions (Unix).
2. Atomically renames the temporary file to the target path.
Updated matrix_state.rs to use this secure writing method.
✅ Verification: Added a unit test in src/persistence/utils.rs verifying that files are created with correct permissions (0o600 on Unix) and content is written correctly. Verified with cargo test persistence::utils.

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PR created automatically by Jules for task 17187642287887974361 started by @kevinaboos

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