fix: notification pushes now match what the 2023 client reads

Most pushes were still shaped for the 2020 watch, so the 2023 client
parsed them as empty or dropped them without any error.

- RelationshipChanged carries the relationship row, to both players, on
  friend request / accept / remove / add and on favorite / mute / ignore.
  Friend requests and accepts also land in the notifications inbox.
- ModerationKick uses the client's flat moderation block, with the
  reason in VoteKickReason (the only field the dialog shows).
- Bans, account deletion, password resets and second-session takeover
  are a Logout with a numeric reason instead of a kick to the dorm; the
  old string reason failed the client's parse.
- SubscribeToPlayers is honoured: each connection joins a presence group
  per subscribed player, and presence (only presence) reaches subscribers
  as well as friends. The hub allows 180 s of client silence, since the
  client reconnects at most 3 times per login.
- Admin broadcast sends its text as a coach message and can start the
  client's own maintenance countdown; ServerMaintenance only reads
  StartsInMinutes.
- ConsumableMapping, StorefrontBalance, gift, player event, level-up,
  RoomUpdate, profile-image and PlayerLeftChat pushes carry the client's
  DTOs. "Join rejected" is the client's RequestGameInvite message.

Contract in docs/recroom-2023-notifications.md; push frames are checked
by NotificationContractTests through a capturing hub context.

DorkNet.Server/Controllers/Chat/ChatController.cs

@@ -460,6 +460,20 @@ public class ChatController(
         var removed = await db.ChatThreadMembers
             .Where(m => m.ThreadKey == chatThreadId && m.PlayerId == Me)
             .ExecuteDeleteAsync();
+        if (removed > 0)
+        {
+            // "PlayerLeftChat" is an Action<ChatMessage> (DLDKCILCKNA.txt:721);
+            // the handler only reads ChatThreadId and SenderPlayerId (the
+            // player who left) and drops them from that thread's members.
+            // Sent to the remaining members, whose member lists otherwise
+            // stayed stale, and to the leaver's own other sessions.
+            var thread = await EnsureThreadRowAsync(chatThreadId);
+            var left = new ChatMessageEntity { ThreadKey = chatThreadId, SenderPlayerId = Me, Body = string.Empty };
+            var recipients = await ThreadRecipientsAsync(chatThreadId);
+            recipients.Add(Me);
+            foreach (var rid in recipients.Distinct())
+                await notifications.NotifyTypedAsync(rid, "PlayerLeftChat", ToWireMessage(left, thread.Id));
+        }
         return Ok(removed > 0 ? ChatSuccess : ChatMembershipNotFound);
     }