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/Services/SystemNotificationService.cs

@@ -28,6 +28,8 @@ public sealed class SystemNotificationService(DorkNetDbContext db, NotificationS
     /// listed; add more as needed.</summary>
     public static class MessageType
     {
+        public const int FriendInvite = 4;
+        public const int RequestGameInvite = 10;
         public const int TextMessage = 30;
         public const int FriendRequestAccepted = 40;
         public const int FriendCodeUsed = 41;
@@ -39,6 +41,8 @@ public sealed class SystemNotificationService(DorkNetDbContext db, NotificationS
         public const int CreatorPublishedNewRoom = 70;
         public const int PlayerAttendingEvent = 80;
         public const int PlayerEventInvitation = 81;
+        public const int CoachMessage = 100;
+        public const int FriendIntroduction = 130;
     }
 
     /// <summary>Persist and push a system notification to