refactor(storage): one definition of task list access (FLAW-20)

accessibleTaskListsSQL in tasklists.go defines which task lists a user
can see; GetTaskLists, GetTaskListByID and the task queries use it
instead of three copies of the same subquery.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/thoughts.go

@@ -131,13 +131,7 @@ func scanThought(rows interface {
 }
 
 func (s *Store) accessibleTaskListIDsQuery() string {
-	return `SELECT id FROM task_lists 
-		   WHERE user_id = ? 
-		   OR id IN (
-		       SELECT task_list_id FROM task_list_shares 
-		       WHERE (target_type = 'user' AND target_id = ?) 
-		          OR (target_type = 'team' AND target_id IN (SELECT team_id FROM team_members WHERE user_id = ?))
-		   )`
+	return accessibleTaskListsSQL
 }
 
 func (s *Store) GetThoughtByID(userID, thoughtID string) (*domain.Thought, error) {