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/lib/task-validation.ts

@@ -17,6 +17,7 @@ export const taskInputSchema = z.object({
   assigneeRole: z.enum(TASK_DEPARTMENTS).nullish(),
   due: z.string().trim().max(120).default("No due date"),
   priority: z.enum(["high", "medium", "low"]).default("medium"),
+  status: z.enum(["todo", "in_progress", "done"]).default("todo"),
   patient: z.string().trim().max(200).nullish(),
   notes: z.string().max(5000).nullish(),
 });