feat(api): add per-chapter requeue scope for dead jobs

Lets the admin manga page requeue a single failed chapter's dead job(s)
inline, without a job id. Adds RequeueScope::Chapter + the matching
request variant and a repo test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/crawler_dead_jobs.rs

@@ -126,6 +126,21 @@ async fn requeue_by_manga_scopes_to_that_manga(pool: PgPool) {
     assert_eq!(state_of(&pool, j2).await, "dead", "other manga untouched");
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn requeue_by_chapter_scopes_to_that_chapter(pool: PgPool) {
+    let (_m, c1) = seed_chapter(&pool, "A", 1).await;
+    let (_m2, c2) = seed_chapter(&pool, "A", 2).await;
+    let j1 = insert_job(&pool, c1, "dead", 5).await;
+    let j2 = insert_job(&pool, c2, "dead", 5).await;
+
+    let n = crawler::requeue_dead_jobs(&pool, RequeueScope::Chapter(c1))
+        .await
+        .unwrap();
+    assert_eq!(n, 1);
+    assert_eq!(state_of(&pool, j1).await, "pending");
+    assert_eq!(state_of(&pool, j2).await, "dead", "other chapter untouched");
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn requeue_single_job(pool: PgPool) {
     let (_m, c1) = seed_chapter(&pool, "A", 1).await;