BUG (PB1): reorder_items only repositioned the items present in the payload,
leaving any omitted item (e.g. one added concurrently in another tab) at its old
position — which then collides with the freshly assigned 0..N-1 range, so
get_playlist's order-by-position returns a nondeterministic/duplicated order.
Now the omitted items get fresh trailing positions (keeping their relative
order), guaranteeing unique contiguous positions.
CLEANUP (PC4): extract _remove_item(db, pl, video_id, mark_dirty=) mirroring the
existing _add_item — remove_watch_later and remove_item duplicated the same
find-by-(playlist,video)/delete/commit block.
Behavior-neutral for the normal full-payload reorder. ruff clean, localdev boots.
ecb2e0b749