feat(tasks): unify saved tasks on ssh_tasks services

- Add shared task_runner.run_saved_task helper used by routers/tasks.py and
  widgets/sources.py SshTaskWidgetSource.
- Saved tasks now target ssh_tasks service instances via default_service_id;
  the legacy default_machine_id and saved_task_runs are removed.
- Actions page lists ssh_tasks services for default and run-time selection.
- Update types, API client, hooks, tests, docs, and changelog.

Backend tests: 222 passed. Frontend lint/build/test: clean (71 passed).

frontend/src/hooks/useSettings.ts

@@ -113,11 +113,11 @@ export function useRunTask() {
 	return useMutation({
 		mutationFn: ({
 			taskId,
-			machineId,
+			serviceId,
 		}: {
 			taskId: string;
-			machineId?: string;
-		}) => runTask(taskId, machineId),
+			serviceId?: string;
+		}) => runTask(taskId, serviceId),
 		onSuccess: () => {
 			queryClient.invalidateQueries({ queryKey: ["tasks"] });
 		},