feat: use exact artist lookup instead of fuzzy search

--artist now calls parser.artist() which resolves the artist name to a
tag id via the /tags/artist/{slug} endpoint, then queries /galleries/tagged
for exact results. BaseParser.artist() defaults to search('artist:...')
as fallback for plugins that don't implement it.

Fixes #430

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

doujinshi_dl/core/plugin.py

@@ -36,6 +36,9 @@ class BaseParser(ABC):
     @abstractmethod
     def search(self, keyword: str, sorting: str = 'date', page=None, **kwargs) -> List[Dict]: ...
 
+    def artist(self, artist_name: str, sorting: str = 'date', page=None, **kwargs) -> List[Dict]:
+        return self.search(f'artist:{artist_name}', sorting=sorting, page=page, **kwargs)
+
     def favorites(self, page=None) -> List[Dict]:
         return []