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/src/repo/crawler.rs

@@ -706,6 +706,8 @@ pub enum RequeueScope {
     All,
     /// Dead jobs whose chapter belongs to this manga.
     Manga(Uuid),
+    /// Dead jobs for a single chapter.
+    Chapter(Uuid),
     /// A single dead job by its id.
     Job(Uuid),
 }
@@ -751,6 +753,18 @@ pub async fn requeue_dead_jobs(pool: &PgPool, scope: RequeueScope) -> sqlx::Resu
             .await?
             .rows_affected()
         }
+        RequeueScope::Chapter(chapter_id) => {
+            sqlx::query(&format!(
+                "UPDATE crawler_jobs {SET} \
+                  WHERE state = 'dead' \
+                    AND (payload->>'chapter_id')::uuid = $1 \
+                    {NO_LIVE_DUP}"
+            ))
+            .bind(chapter_id)
+            .execute(pool)
+            .await?
+            .rows_affected()
+        }
         RequeueScope::Job(job_id) => {
             sqlx::query(&format!(
                 "UPDATE crawler_jobs {SET} WHERE state = 'dead' AND id = $1 {NO_LIVE_DUP}"