Fix empty-category nav and route behavior

Use category-state-driven queries for nav and category listing routes, and add regression tests for empty but valid categories.

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

apps/blog/models.py

@@ -49,10 +49,7 @@ class HomePage(Page):
                 id__in=ArticlePage.objects.live().public().values_list("tags__id", flat=True)
             ).distinct().order_by("name")
         )
-        category_ids = ArticlePage.objects.live().public().values_list("category_id", flat=True)
-        ctx["available_categories"] = Category.objects.filter(show_in_nav=True, id__in=category_ids).order_by(
-            "sort_order", "name"
-        )
+        ctx["available_categories"] = Category.objects.filter(show_in_nav=True).order_by("sort_order", "name")
         return ctx
 
 
@@ -76,9 +73,7 @@ class ArticleIndexPage(RoutablePageMixin, Page):
     def get_listing_context(self, request, active_category=None):
         tag_slug = request.GET.get("tag")
         articles = self.get_articles()
-        all_articles = articles
-        category_ids = all_articles.values_list("category_id", flat=True)
-        available_categories = Category.objects.filter(id__in=category_ids).order_by("sort_order", "name")
+        available_categories = Category.objects.order_by("sort_order", "name")
         category_links = [
             {"category": category, "url": self.get_category_url(category)}
             for category in available_categories
@@ -111,9 +106,7 @@ class ArticleIndexPage(RoutablePageMixin, Page):
 
     @route(r"^category/(?P<category_slug>[-\w]+)/$")
     def category_listing(self, request, category_slug):
-        category_ids = self.get_articles().values_list("category_id", flat=True)
-        category_qs = Category.objects.filter(id__in=category_ids)
-        category = get_object_or_404(category_qs, slug=category_slug)
+        category = get_object_or_404(Category, slug=category_slug)
         return self.render(request, context_overrides=self.get_listing_context(request, active_category=category))
 
     def get_context(self, request, *args, **kwargs):