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/models.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import hashlib
 import re
 from math import ceil
 from typing import Any
@@ -174,20 +175,87 @@ class Category(models.Model):
         return self.name
 
 
+# ── Tag colour palette ────────────────────────────────────────────────────────
+# Deterministic hash-based colour assignment for tags.  Each entry is a dict
+# with Tailwind CSS class strings for bg, text, and border.
+
+TAG_COLOUR_PALETTE: list[dict[str, str]] = [
+    {
+        "bg": "bg-brand-cyan/10",
+        "text": "text-brand-cyan",
+        "border": "border-brand-cyan/20",
+    },
+    {
+        "bg": "bg-brand-pink/10",
+        "text": "text-brand-pink",
+        "border": "border-brand-pink/20",
+    },
+    {
+        "bg": "bg-amber-500/10",
+        "text": "text-amber-400",
+        "border": "border-amber-500/20",
+    },
+    {
+        "bg": "bg-emerald-500/10",
+        "text": "text-emerald-400",
+        "border": "border-emerald-500/20",
+    },
+    {
+        "bg": "bg-violet-500/10",
+        "text": "text-violet-400",
+        "border": "border-violet-500/20",
+    },
+    {
+        "bg": "bg-rose-500/10",
+        "text": "text-rose-400",
+        "border": "border-rose-500/20",
+    },
+    {
+        "bg": "bg-sky-500/10",
+        "text": "text-sky-400",
+        "border": "border-sky-500/20",
+    },
+    {
+        "bg": "bg-lime-500/10",
+        "text": "text-lime-400",
+        "border": "border-lime-500/20",
+    },
+    {
+        "bg": "bg-orange-500/10",
+        "text": "text-orange-400",
+        "border": "border-orange-500/20",
+    },
+    {
+        "bg": "bg-fuchsia-500/10",
+        "text": "text-fuchsia-400",
+        "border": "border-fuchsia-500/20",
+    },
+    {
+        "bg": "bg-teal-500/10",
+        "text": "text-teal-400",
+        "border": "border-teal-500/20",
+    },
+    {
+        "bg": "bg-indigo-500/10",
+        "text": "text-indigo-400",
+        "border": "border-indigo-500/20",
+    },
+]
+
+
+def get_auto_tag_colour_css(tag_name: str) -> dict[str, str]:
+    """Deterministically assign a colour from the palette based on tag name."""
+    digest = hashlib.md5(tag_name.lower().encode(), usedforsecurity=False).hexdigest()  # noqa: S324
+    index = int(digest, 16) % len(TAG_COLOUR_PALETTE)
+    return TAG_COLOUR_PALETTE[index]
+
+
 class TagMetadata(models.Model):
     COLOUR_CHOICES = [("cyan", "Cyan"), ("pink", "Pink"), ("neutral", "Neutral")]
 
     tag = models.OneToOneField("taggit.Tag", on_delete=CASCADE, related_name="metadata")
     colour = models.CharField(max_length=20, choices=COLOUR_CHOICES, default="neutral")
 
-    @classmethod
-    def get_fallback_css(cls) -> dict[str, str]:
-        return {
-            "bg": "bg-zinc-800 dark:bg-zinc-100",
-            "text": "text-white dark:text-black",
-            "border": "border-zinc-600/20 dark:border-zinc-400/20",
-        }
-
     def get_css_classes(self) -> dict[str, str]:
         mapping = {
             "cyan": {
@@ -200,9 +268,13 @@ class TagMetadata(models.Model):
                 "text": "text-brand-pink",
                 "border": "border-brand-pink/20",
             },
-            "neutral": self.get_fallback_css(),
+            "neutral": {
+                "bg": "bg-zinc-800 dark:bg-zinc-100",
+                "text": "text-white dark:text-black",
+                "border": "border-zinc-600/20 dark:border-zinc-400/20",
+            },
         }
-        return mapping.get(self.colour, self.get_fallback_css())
+        return mapping.get(self.colour, get_auto_tag_colour_css(self.tag.name))
 
 
 class ArticlePageAdminForm(WagtailAdminPageForm):
@@ -254,6 +326,8 @@ class ArticlePageAdminForm(WagtailAdminPageForm):
                 cleaned_data.get("body"),
                 max_chars=self.SUMMARY_MAX_CHARS,
             ) or title
+        if not cleaned_data.get("search_description") and cleaned_data.get("summary"):
+            cleaned_data["search_description"] = cleaned_data["summary"]
 
         return cleaned_data
 
@@ -375,6 +449,8 @@ class ArticlePage(SeoMixin, Page):
         return " ".join(parts)
 
     def save(self, *args: Any, **kwargs: Any) -> None:
+        if not getattr(self, "slug", "") and self.title:
+            self.slug = self._auto_slug_from_title()
         if not self.category_id:
             self.category, _ = Category.objects.get_or_create(
                 slug="general",
@@ -382,12 +458,27 @@ class ArticlePage(SeoMixin, Page):
             )
         if not (self.summary or "").strip():
             self.summary = _generate_summary_from_stream(self.body) or self.title
+        if not getattr(self, "search_description", "") and self.summary:
+            self.search_description = self.summary
         if not self.published_date and self.first_published_at:
             self.published_date = self.first_published_at
         if self._should_refresh_read_time():
             self.read_time_mins = self._compute_read_time()
         return super().save(*args, **kwargs)
 
+    def _auto_slug_from_title(self) -> str:
+        base_slug = slugify(self.title) or "article"
+        parent = self.get_parent() if self.pk else None
+        if parent is None:
+            return base_slug
+        sibling_pages = parent.get_children().exclude(pk=self.pk)
+        slug = base_slug
+        suffix = 2
+        while sibling_pages.filter(slug=slug).exists():
+            slug = f"{base_slug}-{suffix}"
+            suffix += 1
+        return slug
+
     def _should_refresh_read_time(self) -> bool:
         if not self.pk:
             return True