feat(chat): free-form GRUPPE channels with mutable participants

- ChatChannelType.GRUPPE: created by Leitungsteam (any KC) or a Gemeinde
  Verantwortliche/r (own KC), mixing team users and guests/Konfis as
  explicit ChatParticipant rows (unlike GEMEINDE_GRUPPE, membership is
  not derived from Gemeinde)
- POST /chat/:kcId/gruppen to create, GET participant-candidates, and
  POST/DELETE /chat/gruppen/:channelId/participants to manage membership
  (creator, LT, or Verantwortliche/r of that KC)
- ChatGateway broadcasts chat:participants-changed on membership change
- PushService updated for nullable ChatParticipant.userId + new
  guestAccountId column
- SyncService now replicates ChatParticipant
- Prisma migration + 14 new unit tests (75/75 passing), tsc clean
- CI: add .gitea/workflows/cybedefend-scan.yml + .cybedefend project config

src/chat/chat.gateway.ts

@@ -106,4 +106,12 @@ export class ChatGateway implements OnGatewayConnection, OnGatewayDisconnect {
       }
     }
   }
+
+  /// Called by ChatController after add/removeParticipant so anyone with the
+  /// channel already open (e.g. the creator's participant-management UI)
+  /// gets a live update. Newly added participants join the room themselves
+  /// via `chat:join` once they open the chat.
+  notifyParticipantsChanged(channelId: string) {
+    this.broadcast(channelId, { event: 'chat:participants-changed', data: { channelId } });
+  }
 }