useWatchlistIdSet: scope subscription snapshot to watchlistKeys.root

[electather]

Part of epic #491. Review nit (B1) on apps/client/src/features/watchlist/hooks/use-watchlist-id-set.ts:23.

Problem

qc.getQueryCache().getAll().length counts every cached query app-wide. Any unrelated query (home feed, artwork, search) triggers re-render of all useWatchlistIdSet consumers → re-runs findAll + useMemo id-set computation even though watchlist data didn't change.

use-is-in-watchlist.ts already has correct pattern in this same PR.

Fix

() =>
  qc
    .getQueryCache()
    .findAll({ queryKey: watchlistKeys.root })
    .reduce((sum, q) => sum + q.state.dataUpdatedAt, 0),

Acceptance criteria

• Snapshot scoped to watchlistKeys.root.
• Snapshot tracks dataUpdatedAt sum so setQueryData + refetch + invalidate refresh advance it.
• Test pins: unrelated query insertion does not trigger snapshot change.
• vp check + vp test pass.