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())