An event card joins and leaves the event

PrivaPub lets a persona join another server's event (owner decision
2026-10-05). The card of a remote event shows the persona's participation
(pending, accepted, rejected) with Join or Leave, except for events by
invitation or taken on another site. Joining is public on the event's
server, so the first join asks the persona once, as reactions and votes do,
remembered apart from them (PublicAct).

The e2e joins the event SocialPub's Mobilizon scenario keeps, checks the
participant on Mobilizon and PrivaPub's answer, and leaves it.

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

Services/StatusService.cs

@@ -172,6 +172,15 @@ namespace decePubClient.Services
 			return await StatusResult(response, cancellationToken);
 		}
 
+		/// <summary>Joins a remote event as the persona (its server is told, and shows who takes part), or leaves it; Data is
+		/// the updated Status, whose event says how the participation stands.</summary>
+		public async Task<WebResult> Participate(string statusId, bool join, CancellationToken cancellationToken = default)
+		{
+			using var response = await httpService.Post(join ? APIs.PrivaPub.POST_M_Join(statusId) : APIs.PrivaPub.POST_M_Leave(statusId),
+				cancellationToken: cancellationToken);
+			return await StatusResult(response, cancellationToken);
+		}
+
 		/// <summary>Votes in a poll as the persona (each option by its index); Data is the updated Poll.</summary>
 		public async Task<WebResult> Vote(string pollId, IReadOnlyCollection<int> choices, CancellationToken cancellationToken = default)
 		{