feat: contract.Rebuilder asks for a debounced rebuild

The sitemap publishes classes.Listing as contract.Rebuilder too, so a
plugin whose Filter reads data of its own can queue the debounced rebuild
a record event queues when that data changes.

classes/listing.go

@@ -20,7 +20,10 @@ type Listing struct {
 	App *backpack.App
 }
 
-var _ contract.Listing = Listing{}
+var (
+	_ contract.Listing   = Listing{}
+	_ contract.Rebuilder = Listing{}
+)
 
 // The page sizes of Entries.
 const (
@@ -39,6 +42,17 @@ func (l Listing) db(ctx context.Context) (*gorm.DB, error) {
 	return db.WithContext(ctx), nil
 }
 
+// RequestRebuild queues one debounced rebuild through
+// RequestDebouncedRebuild, the same one a record event queues
+// (contract.Rebuilder).
+func (l Listing) RequestRebuild(ctx context.Context) error {
+	db, err := l.db(ctx)
+	if err != nil {
+		return err
+	}
+	return RequestDebouncedRebuild(ctx, l.App, db)
+}
+
 // Settings answers the site address, the index address, the default
 // locale and the groups with listed addresses (by key), with their titles
 // (a content type's saved HTML sitemap headings, else the titles of the