feat: implement chat persistence with MongoDB

- Add full chat persistence to database (conversations and messages)
- Implement MongoChatRepository with cursor-based pagination
- Add getConversations/getConversation endpoints in ChatController
- Save user and assistant messages in ChatService.processMessage()
- Track respondedAction (confirm/reject) on proposed event messages
- Load existing messages on chat screen mount
- Add addMessages() bulk action and chatMessageToMessageData() helper to ChatStore
- Add RespondedAction type and UpdateMessageDTO to shared types

apps/client/src/stores/index.ts

@@ -1,3 +1,3 @@
 export { useAuthStore } from "./AuthStore";
-export { useChatStore, type MessageData } from "./ChatStore";
+export { useChatStore, chatMessageToMessageData, type MessageData } from "./ChatStore";
 export { useEventsStore } from "./EventsStore";