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/tests/test_models.py

@@ -2,7 +2,7 @@ import pytest
 from django.db import IntegrityError
 from taggit.models import Tag
 
-from apps.blog.models import ArticleIndexPage, ArticlePage, HomePage, TagMetadata
+from apps.blog.models import ArticleIndexPage, ArticlePage, Category, HomePage, TagMetadata
 from apps.blog.tests.factories import AuthorFactory
 
 
@@ -40,3 +40,29 @@ def test_tag_metadata_css_and_uniqueness():
     assert meta.get_css_classes()["bg"] == "bg-brand-cyan/10"
     with pytest.raises(IntegrityError):
         TagMetadata.objects.create(tag=tag, colour="pink")
+
+
+@pytest.mark.django_db
+def test_article_default_category_is_assigned(home_page):
+    index = ArticleIndexPage(title="Articles", slug="articles")
+    home_page.add_child(instance=index)
+    author = AuthorFactory()
+    article = ArticlePage(
+        title="Categorised",
+        slug="categorised",
+        author=author,
+        summary="s",
+        body=[("rich_text", "<p>body</p>")],
+    )
+    index.add_child(instance=article)
+    article.save()
+    assert article.category.slug == "general"
+
+
+@pytest.mark.django_db
+def test_category_ordering():
+    Category.objects.get_or_create(name="General", slug="general")
+    Category.objects.create(name="Z", slug="z", sort_order=2)
+    Category.objects.create(name="A", slug="a", sort_order=1)
+    names = list(Category.objects.values_list("name", flat=True))
+    assert names == ["General", "A", "Z"]