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/migrations/0002_category_articlepage_category.py

@@ -68,7 +68,7 @@ class Migration(migrations.Migration):
                 blank=True,
                 null=True,
                 on_delete=django.db.models.deletion.PROTECT,
-                related_name="articles",
+                related_name="+",
                 to="blog.category",
             ),
         ),
@@ -79,7 +79,7 @@ class Migration(migrations.Migration):
             name="category",
             field=models.ForeignKey(
                 on_delete=django.db.models.deletion.PROTECT,
-                related_name="articles",
+                related_name="+",
                 to="blog.category",
             ),
         ),