fix(main): list_tags returns ok=true so frontend re-renders pills

Task 12's rewrite of list_tags dropped the ok flag from the response.
loadTags in view-modal.js gates its render on 'if (j.ok)', so every
POST add-tag returned 200, the feedback said 'Tag added', but the
pill list never refreshed because loadTags silently dropped the
response as not-ok.

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

app/main.py

@@ -715,7 +715,7 @@ def list_tags(image_id):
         .order_by(Tag.kind.asc(), Tag.name.asc())
         .all()
     )
-    return jsonify(tags=[
+    return jsonify(ok=True, tags=[
         {
             "id": t.id,
             "name": t.name,