fix: prevent duplicate chapters_idx records + add dedup endpoint

- Fix upsertChapterIdx race: use conflict-retry pattern (mirrors WriteMetadata)
  so concurrent goroutines don't double-POST the same chapter number
- Add DeduplicateChapters to BookWriter interface and Store implementation;
  keeps the latest record per (slug, number) and deletes extras
- Wire POST /api/admin/dedup-chapters/{slug} handler in server.go

backend/internal/bookstore/bookstore.go

@@ -35,6 +35,11 @@ type BookWriter interface {
 
 	// ChapterExists returns true if the markdown object for ref already exists.
 	ChapterExists(ctx context.Context, slug string, ref domain.ChapterRef) bool
+
+	// DeduplicateChapters removes duplicate chapters_idx records for slug,
+	// keeping only one record per chapter number (the one with the latest
+	// updated timestamp). Returns the number of duplicate records deleted.
+	DeduplicateChapters(ctx context.Context, slug string) (int, error)
 }
 
 // BookReader is the read side used by the backend to serve content.