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_models.py

@@ -2,7 +2,15 @@ import pytest
 from django.db import IntegrityError
 from taggit.models import Tag
 
-from apps.blog.models import ArticleIndexPage, ArticlePage, Category, HomePage, TagMetadata
+from apps.blog.models import (
+    TAG_COLOUR_PALETTE,
+    ArticleIndexPage,
+    ArticlePage,
+    Category,
+    HomePage,
+    TagMetadata,
+    get_auto_tag_colour_css,
+)
 from apps.blog.tests.factories import AuthorFactory
 
 
@@ -92,3 +100,108 @@ def test_category_ordering():
     Category.objects.create(name="A", slug="a", sort_order=1)
     names = list(Category.objects.values_list("name", flat=True))
     assert names == ["General", "A", "Z"]
+
+
+# ── Auto tag colour tests ────────────────────────────────────────────────────
+
+
+def test_auto_tag_colour_is_deterministic():
+    """Same tag name always produces the same colour."""
+    css1 = get_auto_tag_colour_css("python")
+    css2 = get_auto_tag_colour_css("python")
+    assert css1 == css2
+
+
+def test_auto_tag_colour_is_case_insensitive():
+    """Tag colour assignment is case-insensitive."""
+    assert get_auto_tag_colour_css("Python") == get_auto_tag_colour_css("python")
+
+
+def test_auto_tag_colour_returns_valid_palette_entry():
+    """Returned CSS dict must be from the palette."""
+    css = get_auto_tag_colour_css("llms")
+    assert css in TAG_COLOUR_PALETTE
+
+
+def test_auto_tag_colour_distributes_across_palette():
+    """Different tag names should map to multiple palette entries."""
+    sample_tags = ["python", "javascript", "rust", "go", "ruby", "java",
+                   "typescript", "css", "html", "sql", "llms", "mlops"]
+    colours = {get_auto_tag_colour_css(t)["text"] for t in sample_tags}
+    assert len(colours) >= 3, "Tags should spread across at least 3 palette colours"
+
+
+@pytest.mark.django_db
+def test_tag_without_metadata_uses_auto_colour():
+    """Tags without TagMetadata should get auto-assigned colour, not neutral."""
+    tag = Tag.objects.create(name="fastapi", slug="fastapi")
+    expected = get_auto_tag_colour_css("fastapi")
+    # Verify no metadata exists
+    assert not TagMetadata.objects.filter(tag=tag).exists()
+    # The template tag helper should fall back to auto colour
+    from apps.core.templatetags.core_tags import _resolve_tag_css
+    assert _resolve_tag_css(tag) == expected
+
+
+@pytest.mark.django_db
+def test_tag_with_metadata_overrides_auto_colour():
+    """Tags with explicit TagMetadata should use that colour."""
+    tag = Tag.objects.create(name="django", slug="django")
+    TagMetadata.objects.create(tag=tag, colour="pink")
+    from apps.core.templatetags.core_tags import _resolve_tag_css
+    css = _resolve_tag_css(tag)
+    assert css["text"] == "text-brand-pink"
+
+
+# ── Auto slug tests ──────────────────────────────────────────────────────────
+
+
+@pytest.mark.django_db
+def test_article_save_auto_generates_slug_from_title(home_page):
+    """Model save should auto-generate slug from title when slug is empty."""
+    index = ArticleIndexPage(title="Articles", slug="articles")
+    home_page.add_child(instance=index)
+    author = AuthorFactory()
+    article = ArticlePage(
+        title="My Great Article",
+        slug="my-great-article",
+        author=author,
+        summary="summary",
+        body=[("rich_text", "<p>body</p>")],
+    )
+    index.add_child(instance=article)
+    article.save()
+    assert article.slug == "my-great-article"
+
+
+@pytest.mark.django_db
+def test_article_save_auto_generates_search_description(article_index):
+    """Model save should populate search_description from summary."""
+    author = AuthorFactory()
+    article = ArticlePage(
+        title="SEO Auto",
+        slug="seo-auto",
+        author=author,
+        summary="This is the article summary.",
+        body=[("rich_text", "<p>body</p>")],
+    )
+    article_index.add_child(instance=article)
+    article.save()
+    assert article.search_description == "This is the article summary."
+
+
+@pytest.mark.django_db
+def test_article_save_preserves_explicit_search_description(article_index):
+    """Explicit search_description should not be overwritten."""
+    author = AuthorFactory()
+    article = ArticlePage(
+        title="SEO Explicit",
+        slug="seo-explicit",
+        author=author,
+        summary="Generated summary.",
+        search_description="Custom SEO description.",
+        body=[("rich_text", "<p>body</p>")],
+    )
+    article_index.add_child(instance=article)
+    article.save()
+    assert article.search_description == "Custom SEO description."