fix: match tag colours to wireframe neon style

Update TagMetadata CSS classes to use brand colours with translucent
backgrounds matching the wireframe design:
- cyan:    bg-brand-cyan/10 text-brand-cyan
- pink:    bg-brand-pink/10 text-brand-pink
- neutral: bg-zinc-800 text-white (dark: bg-zinc-100 text-black)

Previously used muted Tailwind defaults (bg-cyan-100/text-cyan-900)
which appeared as soft pastels instead of the intended neon look.

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

apps/blog/models.py

@@ -100,12 +100,12 @@ class TagMetadata(models.Model):
 
     @classmethod
     def get_fallback_css(cls) -> dict[str, str]:
-        return {"bg": "bg-zinc-100", "text": "text-zinc-800"}
+        return {"bg": "bg-zinc-800 dark:bg-zinc-100", "text": "text-white dark:text-black"}
 
     def get_css_classes(self) -> dict[str, str]:
         mapping = {
-            "cyan": {"bg": "bg-cyan-100", "text": "text-cyan-900"},
-            "pink": {"bg": "bg-pink-100", "text": "text-pink-900"},
+            "cyan": {"bg": "bg-brand-cyan/10", "text": "text-brand-cyan"},
+            "pink": {"bg": "bg-brand-pink/10", "text": "text-brand-pink"},
             "neutral": self.get_fallback_css(),
         }
         return mapping.get(self.colour, self.get_fallback_css())

apps/blog/tests/test_models.py

@@ -37,6 +37,6 @@ def test_article_compute_read_time_excludes_code(home_page):
 def test_tag_metadata_css_and_uniqueness():
     tag = Tag.objects.create(name="llms", slug="llms")
     meta = TagMetadata.objects.create(tag=tag, colour="cyan")
-    assert meta.get_css_classes()["bg"].startswith("bg-cyan")
+    assert meta.get_css_classes()["bg"] == "bg-brand-cyan/10"
     with pytest.raises(IntegrityError):
         TagMetadata.objects.create(tag=tag, colour="pink")