The websocket-client strips the outer {type, data, ...} wrapper before
emitting to handlers, so handlers receive the inner series data object
directly (e.g. {key, name, missing_episodes}) — not {type, data: {...}}.
The series_updated handler was checking data.data which always failed,
causing every update to fall back to a full loadSeries() call instead of
calling updateSingleSeries() directly. This prevented the missing
episodes count from updating in real-time after a download completed.
Fix: check data.key directly and pass data (not data.data) to
updateSingleSeries().
