Function read_and_upsert_file 

fn read_and_upsert_file(
    upsert: &mut Statement<'_>,
    path: &Path,
    path_str: &str,
    size: i64,
    mtime_ns: i64,
    db_path: &Path,
) -> Result<(String, String), CacheError>

Read the file at path, compute its hash, and upsert the row into file_state. Returns the hash and contents.