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/command.py

@@ -86,8 +86,8 @@ def main():
         )
 
     elif options.artist:
-        doujinshis = parser.search(
-            f'artist:{options.artist}',
+        doujinshis = parser.artist(
+            options.artist,
             sorting=options.sorting,
             page=page_list,
             is_page_all=options.page_all,