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/api/client.ts

@@ -245,19 +245,19 @@ export const saveTask = (task: SavedTaskInput) =>
 	});
 export const deleteTask = (taskId: string) =>
 	del<{ status: string }>(`/api/tasks/${encodeURIComponent(taskId)}`);
-export const runTask = (taskId: string, machineId?: string) =>
+export const runTask = (taskId: string, serviceId?: string) =>
 	post<{
 		task_id: string;
 		task_name: string;
-		machine_id: string;
-		machine_name: string;
+		service_id: string;
+		service_name: string;
 		task_type: string;
 		exit_status: number;
 		stdout: string;
 		stderr: string;
 	}>(
-		machineId
-			? `/api/tasks/run?machine_id=${encodeURIComponent(machineId)}`
+		serviceId
+			? `/api/tasks/run?service_id=${encodeURIComponent(serviceId)}`
 			: "/api/tasks/run",
 		{ task_id: taskId },
 	);