- Db struct with rusqlite Connection (WAL mode, foreign keys)
- Tables: task_lists, tasks (with position column), sync_queue
- CRUD: get/insert/update/delete for lists and tasks
- reorder_task shifts positions of sibling tasks
- replace_all_lists and replace_all_tasks for sync import
- push_sync and drain_sync for offline queue management
- All reads sorted by position ASC