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/core/tests/test_more.py

@@ -21,17 +21,20 @@ def test_context_processor_returns_sitesettings(home_page):
 
 
 @pytest.mark.django_db
-def test_get_tag_css_fallback():
+def test_get_tag_css_auto_colour():
+    """Tags without metadata get a deterministic auto-assigned colour."""
     tag = Tag.objects.create(name="x", slug="x")
     value = core_tags.get_tag_css(tag)
-    assert "bg-zinc" in value
+    assert "bg-" in value
+    assert "text-" in value
 
 
 @pytest.mark.django_db
-def test_get_tag_border_css_fallback():
+def test_get_tag_border_css_auto_colour():
+    """Tags without metadata get a deterministic auto-assigned border colour."""
     tag = Tag.objects.create(name="y", slug="y")
     value = core_tags.get_tag_border_css(tag)
-    assert "border-zinc" in value
+    assert "border-" in value
 
 
 @pytest.mark.django_db