Fix mypy relation resolution in CI

Disable reverse manager generation on ArticlePage.category and switch category selection to id-based queries so CI mypy can resolve models reliably.

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

apps/blog/models.py

@@ -49,8 +49,9 @@ class HomePage(Page):
                 id__in=ArticlePage.objects.live().public().values_list("tags__id", flat=True)
             ).distinct().order_by("name")
         )
-        ctx["available_categories"] = (
-            Category.objects.filter(show_in_nav=True, articles__live=True).distinct().order_by("sort_order", "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"
         )
         return ctx
 
@@ -76,9 +77,8 @@ class ArticleIndexPage(RoutablePageMixin, Page):
         tag_slug = request.GET.get("tag")
         articles = self.get_articles()
         all_articles = articles
-        available_categories = (
-            Category.objects.filter(articles__in=all_articles).distinct().order_by("sort_order", "name")
-        )
+        category_ids = all_articles.values_list("category_id", flat=True)
+        available_categories = Category.objects.filter(id__in=category_ids).order_by("sort_order", "name")
         category_links = [
             {"category": category, "url": self.get_category_url(category)}
             for category in available_categories
@@ -111,7 +111,8 @@ class ArticleIndexPage(RoutablePageMixin, Page):
 
     @route(r"^category/(?P<category_slug>[-\w]+)/$")
     def category_listing(self, request, category_slug):
-        category_qs = Category.objects.filter(articles__in=self.get_articles()).distinct()
+        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)
         return self.render(request, context_overrides=self.get_listing_context(request, active_category=category))
 
@@ -187,7 +188,7 @@ class TagMetadata(models.Model):
 
 
 class ArticlePage(SeoMixin, Page):
-    category = models.ForeignKey("blog.Category", on_delete=PROTECT, related_name="articles")
+    category = models.ForeignKey("blog.Category", on_delete=PROTECT, related_name="+")
     author = models.ForeignKey("authors.Author", on_delete=PROTECT)
     hero_image = models.ForeignKey(
         "wagtailimages.Image", null=True, blank=True, on_delete=SET_NULL, related_name="+"