IAV-1323/feat(memory): rolling conversation summary endpoint (Phase 2 memory)
- Add POST /api/izemx-prompt-processing/generate-conversation-summary with ConversationSummaryRequestDTO / ConversationSummaryResponseDTO.
- Implement PromptProcessingService.generateConversationSummary: merge optional previousSummary with excerptToFold via LiteLLM chat completion (factual, bullet-oriented system prompt; language follows excerpt).
- Add Paths.IzemxPromptProcessing.GENERATE_CONVERSATION_SUMMARY and optional LiteLlmProperties.summaryModel (falls back to chatModel when unset).