refactor(import): drop artist:/archive: prefix from constructed tag names

Tag.name holds the bare identifier; Tag.kind already carried the role.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/tasks/import_file.py

@@ -810,10 +810,9 @@ def _add_artist_tag(record: ImageRecord, artist: str):
     if not artist:
         return
 
-    tag_name = f"artist:{artist}"
-    tag = Tag.query.filter_by(name=tag_name).first()
+    tag = Tag.query.filter_by(kind='artist', name=artist).first()
     if not tag:
-        tag = Tag(name=tag_name, kind='artist')
+        tag = Tag(kind='artist', name=artist)
         db.session.add(tag)
         db.session.flush()