Auto slug, auto summary/SEO, and deterministic tag colours

Issue #61: Strengthen auto-generation for slug, summary, and SEO fields.
- ArticlePage.save() now auto-generates slug from title when empty
- ArticlePage.save() auto-populates search_description from summary
- Admin form also auto-populates search_description from summary

Issue #63: Replace manual TagMetadata colour assignment with deterministic
hash-based auto-colour. Tags get a consistent colour from a 12-entry
palette without needing a TagMetadata snippet. TagMetadata still works
as an explicit override.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/blog/tests/test_admin_experience.py

@@ -411,6 +411,73 @@ def test_snippet_category_listing_shows_categories(client, django_user_model):
     assert "Tutorials" in content
 
 
+@pytest.mark.django_db
+def test_article_admin_form_clean_auto_populates_search_description(article_index, django_user_model, monkeypatch):
+    """Form clean should auto-populate search_description from summary."""
+    user = django_user_model.objects.create_user(
+        username="writer",
+        email="writer@example.com",
+        password="writer-pass",
+        first_name="Writer",
+        last_name="User",
+    )
+    form_class = ArticlePage.get_edit_handler().get_form_class()
+    form = form_class(parent_page=article_index, for_user=user)
+
+    body = [
+        SimpleNamespace(block_type="rich_text", value=SimpleNamespace(source="<p>Article body text.</p>")),
+    ]
+    form.cleaned_data = {
+        "title": "SEO Test",
+        "slug": "",
+        "author": None,
+        "category": None,
+        "summary": "",
+        "search_description": "",
+        "body": body,
+    }
+
+    mro = form.__class__.__mro__
+    super_form_class = mro[mro.index(ArticlePageAdminForm) + 1]
+    monkeypatch.setattr(super_form_class, "clean", lambda _self: _self.cleaned_data)
+    cleaned = form.clean()
+
+    assert cleaned["summary"] == "Article body text."
+    assert cleaned["search_description"] == "Article body text."
+
+
+@pytest.mark.django_db
+def test_article_admin_form_preserves_explicit_search_description(article_index, django_user_model, monkeypatch):
+    """Form clean should not overwrite an explicit search_description."""
+    user = django_user_model.objects.create_user(
+        username="writer2",
+        email="writer2@example.com",
+        password="writer-pass",
+    )
+    form_class = ArticlePage.get_edit_handler().get_form_class()
+    form = form_class(parent_page=article_index, for_user=user)
+
+    body = [
+        SimpleNamespace(block_type="rich_text", value=SimpleNamespace(source="<p>Body.</p>")),
+    ]
+    form.cleaned_data = {
+        "title": "SEO Explicit Test",
+        "slug": "seo-explicit-test",
+        "author": None,
+        "category": None,
+        "summary": "My summary.",
+        "search_description": "Custom SEO text.",
+        "body": body,
+    }
+
+    mro = form.__class__.__mro__
+    super_form_class = mro[mro.index(ArticlePageAdminForm) + 1]
+    monkeypatch.setattr(super_form_class, "clean", lambda _self: _self.cleaned_data)
+    cleaned = form.clean()
+
+    assert cleaned["search_description"] == "Custom SEO text."
+
+
 @pytest.mark.django_db
 def test_article_page_omits_admin_messages_on_frontend(article_page, rf):
     """Frontend templates should not render admin session messages."""