fix: add created field to chapters_idx to fix recentlyUpdatedBooks 400

chapters_idx was missing created/updated columns (never defined in the
PocketBase schema), causing PocketBase to return 400 for any query
sorted by -created. recentlyUpdatedBooks() uses this sort.

- Add created date field to chapters_idx schema in pb-init-v3.sh
  (also added via add_field for existing installations)
- Add idx_chapters_idx_created index for sort performance
- Set created timestamp on first insert in upsertChapterIdx so new
  chapters are immediately sortable; existing records retain empty created
  and will sort to the back (acceptable — only affects home page recency)

backend/internal/storage/store.go

@@ -130,7 +130,14 @@ func (s *Store) upsertChapterIdx(ctx context.Context, slug string, ref domain.Ch
 		return err
 	}
 	if len(items) == 0 {
-		postErr := s.pb.post(ctx, "/api/collections/chapters_idx/records", payload, nil)
+		// Set created timestamp on first insert so recentlyUpdatedBooks can sort by it.
+		insertPayload := map[string]any{
+			"slug":    slug,
+			"number":  ref.Number,
+			"title":   ref.Title,
+			"created": time.Now().UTC().Format(time.RFC3339),
+		}
+		postErr := s.pb.post(ctx, "/api/collections/chapters_idx/records", insertPayload, nil)
 		if postErr == nil {
 			return nil
 		}