fix: usage SQL 查询 ambiguous column

l.created_at 缺少表别名前缀导致歧义错误

src/routes/admin/users.rs

@@ -291,7 +291,7 @@ pub async fn get_user_usage(
     let trend: Vec<(String, i64, f64)> = sqlx::query_as(
         r#"
         SELECT
-            strftime('%Y-%m-%d', created_at) as day,
+            strftime('%Y-%m-%d', l.created_at) as day,
             COUNT(*) as request_count,
             SUM(cost) as cost
         FROM usage_logs l