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/api/admin/crawler.rs

@@ -297,6 +297,7 @@ async fn list_dead_jobs(
 enum RequeueRequest {
     All,
     Manga { manga_id: Uuid },
+    Chapter { chapter_id: Uuid },
     Job { job_id: Uuid },
 }
 
@@ -313,6 +314,7 @@ async fn requeue_dead_jobs(
     let scope = match &body {
         RequeueRequest::All => RequeueScope::All,
         RequeueRequest::Manga { manga_id } => RequeueScope::Manga(*manga_id),
+        RequeueRequest::Chapter { chapter_id } => RequeueScope::Chapter(*chapter_id),
         RequeueRequest::Job { job_id } => RequeueScope::Job(*job_id),
     };
     let requeued = repo::crawler::requeue_dead_jobs(&state.db, scope).await?;
@@ -332,6 +334,7 @@ fn scope_label(r: &RequeueRequest) -> &'static str {
     match r {
         RequeueRequest::All => "all",
         RequeueRequest::Manga { .. } => "manga",
+        RequeueRequest::Chapter { .. } => "chapter",
         RequeueRequest::Job { .. } => "job",
     }
 }