Add category taxonomy and navigation integration

Implements Issue #35 with category snippets, article category routing, category-aware templates, and category RSS feeds with tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/blog/models.py

@@ -7,10 +7,12 @@ from typing import Any
 from django.core.paginator import EmptyPage, PageNotAnInteger, Paginator
 from django.db import models
 from django.db.models import CASCADE, PROTECT, SET_NULL, Prefetch
+from django.shortcuts import get_object_or_404
 from modelcluster.contrib.taggit import ClusterTaggableManager
 from modelcluster.fields import ParentalKey
 from taggit.models import Tag, TaggedItemBase
 from wagtail.admin.panels import FieldPanel, PageChooserPanel
+from wagtail.contrib.routable_page.models import RoutablePageMixin, route
 from wagtail.fields import RichTextField, StreamField
 from wagtail.models import Page
 from wagtailseo.models import SeoMixin
@@ -34,7 +36,7 @@ class HomePage(Page):
         articles_qs = (
             ArticlePage.objects.live()
             .public()
-            .select_related("author")
+            .select_related("author", "category")
             .prefetch_related("tags__metadata")
             .order_by("-first_published_at")
         )
@@ -47,10 +49,13 @@ class HomePage(Page):
                 id__in=ArticlePage.objects.live().public().values_list("tags__id", flat=True)
             ).distinct().order_by("name")
         )
+        ctx["available_categories"] = (
+            Category.objects.filter(show_in_nav=True, articles__live=True).distinct().order_by("sort_order", "name")
+        )
         return ctx
 
 
-class ArticleIndexPage(Page):
+class ArticleIndexPage(RoutablePageMixin, Page):
     parent_page_types = ["blog.HomePage"]
     subpage_types = ["blog.ArticlePage"]
     ARTICLES_PER_PAGE = 12
@@ -59,15 +64,24 @@ class ArticleIndexPage(Page):
         return (
             ArticlePage.objects.child_of(self)
             .live()
-            .select_related("author")
+            .select_related("author", "category")
             .prefetch_related("tags__metadata")
             .order_by("-first_published_at")
         )
 
-    def get_context(self, request, *args, **kwargs):
-        ctx = super().get_context(request, *args, **kwargs)
+    def get_category_url(self, category):
+        return f"{self.url}category/{category.slug}/"
+
+    def get_listing_context(self, request, active_category=None):
         tag_slug = request.GET.get("tag")
         articles = self.get_articles()
+        all_articles = articles
+        available_categories = (
+            Category.objects.filter(articles__in=all_articles).distinct().order_by("sort_order", "name")
+        )
+        category_links = [{"category": category, "url": self.get_category_url(category)} for category in available_categories]
+        if active_category:
+            articles = articles.filter(category=active_category)
         available_tags = (
             Tag.objects.filter(id__in=articles.values_list("tags__id", flat=True)).distinct().order_by("name")
         )
@@ -81,10 +95,25 @@ class ArticleIndexPage(Page):
             page_obj = paginator.page(1)
         except EmptyPage:
             page_obj = paginator.page(paginator.num_pages)
-        ctx["articles"] = page_obj
-        ctx["paginator"] = paginator
-        ctx["active_tag"] = tag_slug
-        ctx["available_tags"] = available_tags
+        return {
+            "articles": page_obj,
+            "paginator": paginator,
+            "active_tag": tag_slug,
+            "available_tags": available_tags,
+            "available_categories": available_categories,
+            "category_links": category_links,
+            "active_category": active_category,
+            "active_category_url": self.get_category_url(active_category) if active_category else "",
+        }
+
+    @route(r"^category/(?P<category_slug>[-\w]+)/$")
+    def category_listing(self, request, category_slug):
+        category = get_object_or_404(Category.objects.filter(articles__in=self.get_articles()).distinct(), slug=category_slug)
+        return self.render(request, context_overrides=self.get_listing_context(request, active_category=category))
+
+    def get_context(self, request, *args, **kwargs):
+        ctx = super().get_context(request, *args, **kwargs)
+        ctx.update(self.get_listing_context(request))
         return ctx
 
 
@@ -92,6 +121,36 @@ class ArticleTag(TaggedItemBase):
     content_object = ParentalKey("blog.ArticlePage", related_name="tagged_items", on_delete=CASCADE)
 
 
+class Category(models.Model):
+    COLOUR_CHOICES = [("cyan", "Cyan"), ("pink", "Pink"), ("neutral", "Neutral")]
+
+    name = models.CharField(max_length=100, unique=True)
+    slug = models.SlugField(unique=True)
+    description = models.TextField(blank=True)
+    hero_image = models.ForeignKey(
+        "wagtailimages.Image", null=True, blank=True, on_delete=SET_NULL, related_name="+"
+    )
+    colour = models.CharField(max_length=20, choices=COLOUR_CHOICES, default="neutral")
+    sort_order = models.IntegerField(default=0)
+    show_in_nav = models.BooleanField(default=True)
+
+    panels = [
+        FieldPanel("name"),
+        FieldPanel("slug"),
+        FieldPanel("description"),
+        FieldPanel("hero_image"),
+        FieldPanel("colour"),
+        FieldPanel("sort_order"),
+        FieldPanel("show_in_nav"),
+    ]
+
+    class Meta:
+        ordering = ["sort_order", "name"]
+
+    def __str__(self):
+        return self.name
+
+
 class TagMetadata(models.Model):
     COLOUR_CHOICES = [("cyan", "Cyan"), ("pink", "Pink"), ("neutral", "Neutral")]
 
@@ -124,6 +183,7 @@ class TagMetadata(models.Model):
 
 
 class ArticlePage(SeoMixin, Page):
+    category = models.ForeignKey("blog.Category", on_delete=PROTECT, related_name="articles")
     author = models.ForeignKey("authors.Author", on_delete=PROTECT)
     hero_image = models.ForeignKey(
         "wagtailimages.Image", null=True, blank=True, on_delete=SET_NULL, related_name="+"
@@ -138,6 +198,7 @@ class ArticlePage(SeoMixin, Page):
     subpage_types: list[str] = []
 
     content_panels = Page.content_panels + [
+        FieldPanel("category"),
         FieldPanel("author"),
         FieldPanel("hero_image"),
         FieldPanel("summary"),
@@ -151,6 +212,11 @@ class ArticlePage(SeoMixin, Page):
     search_fields = Page.search_fields
 
     def save(self, *args: Any, **kwargs: Any) -> None:
+        if not self.category_id:
+            self.category, _ = Category.objects.get_or_create(
+                slug="general",
+                defaults={"name": "General", "description": "General articles", "colour": "neutral"},
+            )
         self.read_time_mins = self._compute_read_time()
         return super().save(*args, **kwargs)