Add created_at/updated_at to Task model, DB, and API

- Add created_at and updated_at fields to Task struct
- Preserve existing created_at on upsert in insert_task
- Parse updated field from Google Tasks API response
- Add created_at column to DB schema with migration

src/main.rs

@@ -255,6 +255,8 @@ async fn push_sync(
                     status: TaskStatus::NeedsAction,
                     due: None,
                     position: 0,
+                    created_at: None,
+                    updated_at: None,
                 });
                 api.create_task(&item.list_id, &task).await
             }
@@ -267,6 +269,8 @@ async fn push_sync(
                     status: TaskStatus::NeedsAction,
                     due: None,
                     position: 0,
+                    created_at: None,
+                    updated_at: None,
                 });
                 api.update_task(&item.list_id, &task).await
             }