test(04-04): add failing tests for item CRUD endpoints + rank helpers
- Add rank.test.ts: unit tests for rankForAppend/rankBetween (RED - no impl yet)
- Extend lists.test.ts with item route tests: POST /:id/items, GET /:id/items,
PATCH /list-items/:id (per-field LWW D-08), DELETE /list-items/:id (D-09)
- Import listItems from schema; add seedItem helper
- Tests cover: fractional rank assignment (D-13), exact-one-field refine (T-04-07),
uncheck rank recompute, access gating T-04-05, delete-wins no resurrection D-09