feat: add nullable owner_id columns to lists and tasks

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/utils/schema.ts

@@ -20,4 +20,8 @@ create table if not exists tasks (
 
 create index if not exists idx_tasks_list_id on tasks(list_id);
 create index if not exists idx_tasks_unconsumed on tasks(consumed) where consumed = false;
+
+-- Multi-user plumbing: rows are owned by a Zitadel sub. NULL = legacy/unowned (pre-multi-user).
+alter table lists add column if not exists owner_id text;
+alter table tasks add column if not exists owner_id text;
 `;