Bursts of completions (e.g. small playlist entries) fire one item_moved
frame per item. Route item_moved through the same ItemBatcher mechanism
as item_updated: one frame per 500ms tick, payload always a list.
ItemBatcher gains an optional key callable since item_moved payloads are
{"to", "preset", "item"} dicts rather than ItemDTOs; moves coalesce by
item _id with last write wins, so an item that moves twice within a tick
delivers only its final destination. The notifications service consumes
ITEM_MOVED from the event bus directly and is unaffected.
BREAKING CHANGE: the item_moved WebSocket event payload (data) is now
always an array of {to, preset, item} objects instead of a single one.
