refactor(todo): 移除 due_at 字段，改用 order 字段管理象限内顺序

docs/protocols/models/todo.md

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+# Todo Data Protocol
+
+## Scope
+
+Defines the backend/frontend data contract for `/api/v1/todos`.
+
+## Field Definitions
+
+- `id`: string (UUID)
+- `owner_id`: string (UUID)
+- `title`: string, `1..255`
+- `description`: string or `null`, max `1000`
+- `priority`: int, quadrant value in `[1, 4]`
+- `order`: int, zero-based ordering inside the same `priority` bucket, `>= 0`
+- `status`: enum string: `pending | done | canceled`
+- `completed_at`: datetime string or `null`
+- `created_at`: datetime string
+- `updated_at`: datetime string
+- `schedule_items`: array
+
+## Request Contracts
+
+### Create Todo (`POST /api/v1/todos`)
+
+- required: `title`
+- optional: `description`, `priority`, `order`, `schedule_item_ids`
+- `order` omitted: backend assigns append position in the target quadrant.
+
+### Update Todo (`PATCH /api/v1/todos/{todo_id}`)
+
+- optional: `title`, `description`, `priority`, `order`, `status`, `schedule_item_ids`
+- `order` is interpreted inside the todo's final `priority` quadrant.
+
+## Ordering Rules
+
+- Todo list API returns items sorted by `priority ASC`, then `order ASC`.
+- Drag reorder in same quadrant updates `order` to a continuous sequence starting from `0`.
+- Drag move across quadrants updates both `priority` and `order`, and source/target quadrants should both stay contiguous from `0`.
+
+## Compatibility Notes
+
+- `due_at` is removed from todo protocol.
+- Clients must not send or depend on `due_at` for todo ordering.