pub fn parse_datetime(s: &str) -> Option<DateTime<Utc>>
Parse a date/datetime string flexibly, accepting both full RFC 3339 timestamps (e.g., 2023-01-01T00:00:00Z) and plain ISO 8601 dates (e.g., 2023-01-01).
2023-01-01T00:00:00Z
2023-01-01