server/posts: add relationCount field

server/szurubooru/func/posts.py

@@ -109,6 +109,7 @@ def serialize_post(post, authenticated_user, options=None):
             'favoriteCount': lambda: post.favorite_count,
             'commentCount': lambda: post.comment_count,
             'noteCount': lambda: post.note_count,
+            'relationCount': lambda: post.relation_count,
             'featureCount': lambda: post.feature_count,
             'lastFeatureTime': lambda: post.last_feature_time,
             'favoritedBy': lambda: [

server/szurubooru/tests/func/test_posts.py

@@ -154,6 +154,7 @@ def test_serialize_post(
         'commentCount': 2,
         'noteCount': 0,
         'featureCount': 1,
+        'relationCount': 0,
         'lastFeatureTime': datetime.datetime(1999, 1, 1),
         'favoritedBy': ['fav1'],
         'hasCustomThumbnail': True,