Fix ugoira zip conversion — convert cached zips before returning

Previously cached .zip files passed the valid media check and were
returned without conversion. Now zips are detected and converted
to GIF before the general cache check.

booru_viewer/core/cache.py

@@ -91,6 +91,9 @@ async def download_image(
         gif_path = local.with_suffix(".gif")
         if gif_path.exists():
             return gif_path
+        # If the zip is cached but not yet converted, convert it now
+        if local.exists() and zipfile.is_zipfile(local):
+            return _convert_ugoira_to_gif(local)
 
     # Validate cached file isn't corrupt (e.g. HTML error page saved as image)
     if local.exists():