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/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")