Add missing DELETE /api/kanban/tasks/{id} endpoint

The Kanban detail modal's Delete button called api.deleteKanbanTask(),
which didn't exist on the client, and there was no backend route for
it either - clicking Delete just threw 'api.deleteKanbanTask is not
a function'. Add both the client method and the backend endpoint.

dashboard/api.js

@@ -55,6 +55,7 @@ const api = {
   // Kanban
   getKanbanBoard: (status) => api.get(status ? `/api/kanban/board?status=${encodeURIComponent(status)}` : '/api/kanban/board'),
   getKanbanTask: (id) => api.get(`/api/kanban/tasks/${encodeURIComponent(id)}`),
+  deleteKanbanTask: (id) => api.del(`/api/kanban/tasks/${encodeURIComponent(id)}`),
   createKanbanTask: (data) => api.post('/api/kanban/tasks', data),
   updateKanbanTask: (id, data) => api.patch(`/api/kanban/tasks/${encodeURIComponent(id)}`, data),
   completeKanbanTask: (id, summary) => api.post(`/api/kanban/tasks/${encodeURIComponent(id)}/complete`, { summary }),

server.py

@@ -947,6 +947,15 @@ def kanban_get_task(task_id: str):
         raise HTTPException(404, "Task not found")
     return json.loads(path.read_text())
 
+@app.delete("/api/kanban/tasks/{task_id}")
+def kanban_delete_task(task_id: str):
+    path = kanban_task_path(task_id)
+    if not path.exists():
+        raise HTTPException(404, "Task not found")
+    path.unlink()
+    append_audit({"action": "kanban_task_deleted", "task_id": task_id})
+    return {"status": "deleted", "task_id": task_id}
+
 @app.post("/api/kanban/tasks")
 def kanban_create_task(data: KanbanTaskCreate):
     try: