IAV-257/ implement conversation management service with full CRUD and history

• Add createConversation with ticket association and N8N webhook forwarding
• Add createConversation overload for N8N inbound request mapping
• Add createConversationWithoutTicket for agent AI direct conversations
• Add getConversation by UID and getConversationsByTicket
• Add getConversationHistory with subagentId, channel, and search filtering
• Add getMessagesByClient for paginated message retrieval by chatId
• Add ConversationDto, ConversationN8nRequestDto, ConversationAgentAiRequestDto
• Add ConversationThreadDTO and ConversationDetailDTO projection DTOs
• Add REST controllers for all conversation endpoints
• Add WebSocket broadcasting on conversation creation

Closes IAV-257