feat(tasks): kanban board with To Do / In Progress / Done columns

- add a `status` field to tasks (backend schema/validation/service/types
  + frontend types), kept in sync with the legacy `done` flag
- migration 0017 adds the column and backfills done tasks to "done"
- rewrite the tasks page as a three-column board: per-column add buttons,
  draggable cards, and a status mover in the detail sheet

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/types/task.ts

@@ -2,6 +2,8 @@
 // `lib/tasks.ts` Task type. Scoped to the active clinic (a shared care-team
 // to-do board). `patient` is an optional free-text reference for context.
 export type TaskPriority = "high" | "medium" | "low";
+// Board column the task sits in. `done` mirrors `status === "done"`.
+export type TaskStatus = "todo" | "in_progress" | "done";
 
 export type Task = {
   id: string;
@@ -11,6 +13,7 @@ export type Task = {
   assigneeRole: string | null;
   due: string;
   priority: TaskPriority;
+  status: TaskStatus;
   patient: string | null;
   notes: string | null;
   done: boolean;