todos: a backburner column, and the parked items on it

backburner joins TODO_STATUSES and the board's columns; the three items in the
backburner theme move onto the status, so parked is now a lane rather than a
folder you have to remember to open.

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

src/servers/api/todos/todo-tree.ts

@@ -16,7 +16,7 @@ import { join, resolve } from 'node:path';
 /** Resolved per call, not at module load: the server asserts its cwd is the repo root at boot. */
 const todoDir = () => resolve(process.cwd(), 'todo');
 
-export const TODO_STATUSES = ['open', 'doing', 'blocked', 'done'] as const;
+export const TODO_STATUSES = ['open', 'doing', 'blocked', 'backburner', 'done'] as const;
 export type TodoStatus = (typeof TODO_STATUSES)[number];
 
 export type TodoItem = {