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/dto/create-channel.dto.ts

@@ -1,4 +1,4 @@
-import { IsEnum, IsOptional, IsString } from 'class-validator';
+import { ArrayUnique, IsArray, IsEnum, IsOptional, IsString } from 'class-validator';
 import { ChatChannelType } from '@prisma/client';
 
 export class CreateChannelDto {
@@ -8,4 +8,24 @@ export class CreateChannelDto {
   @IsOptional()
   @IsString()
   gemeindeId?: string;
+
+  /// Display name; used for GRUPPE channels.
+  @IsOptional()
+  @IsString()
+  name?: string;
+
+  /// Initial participants for a GRUPPE channel (team users). More can be
+  /// added/removed later via the participants endpoints.
+  @IsOptional()
+  @IsArray()
+  @ArrayUnique()
+  @IsString({ each: true })
+  participantUserIds?: string[];
+
+  /// Initial guest/Konfi participants for a GRUPPE channel.
+  @IsOptional()
+  @IsArray()
+  @ArrayUnique()
+  @IsString({ each: true })
+  participantGuestIds?: string[];
 }