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/ChatStore.ts

@@ -1,5 +1,5 @@
 import { create } from "zustand";
-import { ProposedEventChange } from "@caldav/shared";
+import { ChatMessage, ProposedEventChange } from "@caldav/shared";
 
 type BubbleSide = "left" | "right";
 
@@ -14,6 +14,7 @@ export type MessageData = {
 
 interface ChatState {
   messages: MessageData[];
+  addMessages: (messages: MessageData[]) => void;
   addMessage: (message: MessageData) => void;
   updateMessage: (id: string, updates: Partial<MessageData>) => void;
   clearMessages: () => void;
@@ -21,6 +22,9 @@ interface ChatState {
 
 export const useChatStore = create<ChatState>((set) => ({
   messages: [],
+  addMessages(messages) {
+    set((state) => ({messages: [...state.messages, ...messages]}))
+  },
   addMessage: (message: MessageData) => {
     set((state) => ({ messages: [...state.messages, message] }));
   },
@@ -35,3 +39,15 @@ export const useChatStore = create<ChatState>((set) => ({
     set({ messages: [] });
   },
 }));
+
+// Helper to convert server ChatMessage to client MessageData
+export function chatMessageToMessageData(msg: ChatMessage): MessageData {
+  return {
+    id: msg.id,
+    side: msg.sender === "assistant" ? "left" : "right",
+    content: msg.content,
+    proposedChange: msg.proposedChange,
+    respondedAction: msg.respondedAction,
+    conversationId: msg.conversationId,
+  };
+}