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/domain/models.rs

@@ -16,6 +16,8 @@ pub struct Task {
     pub status: TaskStatus,
     pub due: Option<NaiveDateTime>,
     pub position: i64,
+    pub created_at: Option<NaiveDateTime>,
+    pub updated_at: Option<NaiveDateTime>,
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]