The per-download 500ms throttle still scales linearly with concurrency:
N active downloads produce up to 2N item_updated frames per second, each
with its own serialization pass and per-client send.
Add an ItemBatcher at the WebSocket boundary that coalesces dirty items
by _id (last write wins) and emits one item_updated frame per 500ms
tick whose payload is always a list of items, even for a single update.
Leading-edge emits keep isolated updates (e.g. renames via the API)
instant, and a trailing flush at most 500ms later delivers the rest.
Pending items are flushed on shutdown before clients disconnect.
Emitter call sites, the EventBus, and the StatusTracker throttle are
unchanged; the batcher only changes the wire format. The frontend now
types item_updated as StoreItem[] and iterates the array.
BREAKING CHANGE: the item_updated WebSocket event payload (data) is now
always an array of items instead of a single item object.
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