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/drizzle/0017_wealthy_northstar.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "tasks" ADD COLUMN "status" text DEFAULT 'todo' NOT NULL;
+--> statement-breakpoint
+UPDATE "tasks" SET "status" = 'done' WHERE "done" = true;