Add category taxonomy and navigation integration

Implements Issue #35 with category snippets, article category routing, category-aware templates, and category RSS feeds with tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/blog/wagtail_hooks.py

@@ -1,7 +1,7 @@
 from wagtail.snippets.models import register_snippet
 from wagtail.snippets.views.snippets import SnippetViewSet
 
-from apps.blog.models import TagMetadata
+from apps.blog.models import Category, TagMetadata
 
 
 class TagMetadataViewSet(SnippetViewSet):
@@ -11,3 +11,14 @@ class TagMetadataViewSet(SnippetViewSet):
 
 
 register_snippet(TagMetadataViewSet)
+
+
+class CategoryViewSet(SnippetViewSet):
+    model = Category
+    icon = "folder-open-inverse"
+    list_display = ["name", "slug", "show_in_nav", "sort_order"]
+    list_filter = ["show_in_nav"]
+    ordering = ["sort_order", "name"]
+
+
+register_snippet(CategoryViewSet)