LoadForListAsync loaded the entire tasks table (all lists, two Include
joins) then filtered to the selected list in C#, never using
idx_tasks_list_id/idx_tasks_status. ITaskListFilter now exposes
MatchExpression, an Expression<Func<TaskEntity,bool>> that EF Core can
translate to SQL; a new TaskListFilterBase compiles it on demand for the
existing in-memory Matches(). The primary match query now runs as
db.Tasks.Where(filter.MatchExpression), with two small follow-up queries
(scoped by parent-id IN-lists) for contextual planning-parent rows and
pulled-in children, followed by an explicit re-sort to restore the global
SortOrder/CreatedAt ordering Regroup() depends on.