Function emit_progress 

pub fn emit_progress<E: EventEmitter>(
    channel: &E,
    request_id: &str,
    bytes_done: u64,
    bytes_total: Option<u64>,
    parts_done: u32,
    parts_total: u32,
    throttle: &mut ProgressThrottle,
    now_ms: i64,
) -> Result<bool, AppError>

Emit a transfer:progress event if the throttle gate is open.

Updates throttle on emission. now_ms is the current wall-clock time in milliseconds (callers obtain this via std::time::SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_millis() as i64).

Returns Ok(true) when an event was emitted, Ok(false) when throttled.