refactor: use Box<dyn Error> instead of anyhow

For a simple binary, std::error::Error is sufficient.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kenkoooo

atcoder-problems-backend/Cargo.toml

@@ -4,7 +4,6 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-anyhow.workspace = true
 chrono.workspace = true
 crawler.workspace = true
 rand.workspace = true

atcoder-problems-backend/src/bin/dump-json.rs

@@ -1,12 +1,14 @@
-use anyhow::Result;
 use s3::S3Client;
 use sea_orm::{Database, DatabaseConnection, EntityTrait, QueryOrder};
 use serde::Serialize;
 use std::cmp::Reverse;
 use std::collections::{BTreeMap, HashMap};
+use std::error::Error;
 
 const LANGUAGE_COUNT_LIMIT: usize = 1000;
 
+type Result<T> = std::result::Result<T, Box<dyn Error>>;
+
 #[tokio::main]
 async fn main() -> Result<()> {
     tracing_subscriber::fmt()