feat(messaging): add in-app messaging module with Conversation + Message models

Implements buyer-agent in-app messaging (Task 8.4):
- Prisma models: Conversation, ConversationParticipant, Message
- Full DDD module: domain entities, repository interfaces, CQRS commands/queries
- REST API: POST/GET conversations, POST/GET messages, PATCH read, DELETE messages
- WebSocket gateway (/messaging namespace): real-time message delivery, typing indicators, room-based routing
- 46 unit tests covering handlers, repositories, controller, and gateway

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/modules/messaging/infrastructure/index.ts

@@ -0,0 +1,2 @@
+export { PrismaConversationRepository } from './repositories/prisma-conversation.repository';
+export { PrismaMessageRepository } from './repositories/prisma-message.repository';