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).

backend/tests/test_widgets.py

@@ -359,7 +359,7 @@ async def test_ssh_task_adapter_records_history_on_run(client):
             "task_type": "shell",
             "content": "echo hi",
             "enabled": True,
-            "default_machine_id": "",
+            "default_service_id": "",
         }
     )
     service = store.upsert_service(
@@ -369,6 +369,7 @@ async def test_ssh_task_adapter_records_history_on_run(client):
     fake_result = SimpleNamespace(exit_status=0, stdout="hi\n", stderr="")
     fake_client = SimpleNamespace(run=lambda *a, **k: fake_result)
 
+    from media_library_viewer_api.services.task_runner import build_ssh_client
     from media_library_viewer_api.widgets.sources import SshTaskWidgetSource
 
     adapter = SshTaskWidgetSource()
@@ -377,7 +378,7 @@ async def test_ssh_task_adapter_records_history_on_run(client):
     )
     with (
         patch("media_library_viewer_api.widgets.sources.get_settings_store", return_value=store),
-        patch("media_library_viewer_api.widgets.sources._build_ssh_client", return_value=fake_client),
+        patch("media_library_viewer_api.services.task_runner.build_ssh_client", return_value=fake_client),
     ):
         result = await adapter.fetch(service_record, "task_output", {"task_id": task["id"]})