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_more_models.py

@@ -1,7 +1,5 @@
 import pytest
 
-from apps.blog.models import TagMetadata
-
 
 @pytest.mark.django_db
 def test_home_context_lists_articles(home_page, article_page):
@@ -22,6 +20,7 @@ def test_get_related_articles_fallback(article_page, article_index):
     assert isinstance(related, list)
 
 
-def test_tag_metadata_fallback_classes():
-    css = TagMetadata.get_fallback_css()
+def test_auto_tag_colour_returns_valid_css():
+    from apps.blog.models import get_auto_tag_colour_css
+    css = get_auto_tag_colour_css("test-tag")
     assert css["bg"].startswith("bg-")