M2 checklists: note kind + items backend + editor/card UI

- Migration 0006: notes.kind ('text'|'list') + note_items (text, checked,
  position). Item API: add/update(toggle)/delete/reorder; PATCH note kind; note
  responses include kind + items[] (merged in one query alongside labels).
- notes store: kind/items on Note, setKind/addItem/updateItem/deleteItem.
- NoteChecklist component (toggle/add/edit/delete items); rendered read-only-ish
  on cards (checkboxes toggle) and editable in the editor.
- Editor: convert text<->checklist (body lines become items on convert to list).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

src/thoughtsync/models/all.py

@@ -3,4 +3,4 @@
 Imported for side effects only (model registration on Base.metadata).
 """
 
-from . import group, label, note, settings, share, user  # noqa: F401
+from . import group, label, note, note_item, settings, share, user  # noqa: F401