A notification told alone names its group

v1 notifications, and those a stream tells, carried an 'ungrouped' key, so
a client reading grouped notifications could not fold a new like into the
group of likes it already shows. Each now names the group it belongs to by
default (likes and boosts of one post, follows within an hour).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub.Tests/Http/OneAnswerTests.cs

@@ -167,6 +167,8 @@ namespace PrivaPub.Tests.Http
 			var key = group.Text("group_key");
 
 			Assert.Equal($"grouped-favourite-{id}", key);
+			// told alone (v1, a stream), each notification names the group it belongs to
+			Assert.All((await author.Client.Get("/api/v1/notifications?types[]=favourite")).Ok().Array, n => Assert.Equal(key, n.Text("group_key")));
 			Assert.Equal(2, group.Number("notifications_count"));
 			Assert.Equal(2, group["sample_account_ids"]!.AsArray().Count);
 			Assert.Equal(id, group.Text("status_id"));