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/templatetags/core_tags.py

@@ -4,7 +4,7 @@ from django import template
 from django.utils.safestring import mark_safe
 from wagtail.models import Site
 
-from apps.blog.models import ArticleIndexPage, Category, TagMetadata
+from apps.blog.models import ArticleIndexPage, Category, TagMetadata, get_auto_tag_colour_css
 from apps.core.models import SiteSettings
 from apps.legal.models import LegalPage
 
@@ -70,20 +70,24 @@ def get_categories_nav(context):
     ]
 
 
-@register.simple_tag
-@register.filter
-def get_tag_css(tag):
+def _resolve_tag_css(tag) -> dict[str, str]:
+    """Return CSS classes for a tag, using TagMetadata if set, else auto-colour."""
     meta = getattr(tag, "metadata", None)
     if meta is None:
         meta = TagMetadata.objects.filter(tag=tag).first()
-    classes = meta.get_css_classes() if meta else TagMetadata.get_fallback_css()
+    if meta:
+        return meta.get_css_classes()
+    return get_auto_tag_colour_css(tag.name)
+
+
+@register.simple_tag
+@register.filter
+def get_tag_css(tag):
+    classes = _resolve_tag_css(tag)
     return mark_safe(f"{classes['bg']} {classes['text']}")
 
 
 @register.filter
 def get_tag_border_css(tag):
-    meta = getattr(tag, "metadata", None)
-    if meta is None:
-        meta = TagMetadata.objects.filter(tag=tag).first()
-    classes = meta.get_css_classes() if meta else TagMetadata.get_fallback_css()
+    classes = _resolve_tag_css(tag)
     return mark_safe(classes.get("border", ""))

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