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/app.rs

@@ -537,6 +537,8 @@ impl App {
                             status: TaskStatus::NeedsAction,
                             due: None,
                             position: 0,
+                            created_at: None,
+                            updated_at: None,
                         };
                         self.db.insert_task(&task).ok();
                         self.db.push_sync(