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/core/tests/test_tags.py

@@ -1,5 +1,7 @@
 import pytest
 
+from apps.blog.models import ArticleIndexPage, ArticlePage, Category
+from apps.blog.tests.factories import AuthorFactory
 from apps.legal.models import LegalIndexPage, LegalPage
 
 
@@ -13,3 +15,25 @@ def test_get_legal_pages_tag(client, home_page):
 
     resp = client.get("/")
     assert resp.status_code == 200
+
+
+@pytest.mark.django_db
+def test_categories_nav_tag_renders_category_link(client, home_page):
+    index = ArticleIndexPage(title="Articles", slug="articles")
+    home_page.add_child(instance=index)
+    category = Category.objects.create(name="Reviews", slug="reviews", show_in_nav=True)
+    author = AuthorFactory()
+    article = ArticlePage(
+        title="R1",
+        slug="r1",
+        author=author,
+        summary="summary",
+        body=[("rich_text", "<p>body</p>")],
+        category=category,
+    )
+    index.add_child(instance=article)
+    article.save_revision().publish()
+
+    resp = client.get("/")
+    assert resp.status_code == 200
+    assert "/articles/category/reviews/" in resp.content.decode()