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/server/src/services/interfaces/ChatRepository.ts

@@ -3,6 +3,7 @@ import {
   Conversation,
   CreateMessageDTO,
   GetMessagesOptions,
+  UpdateMessageDTO,
 } from "@caldav/shared";
 
 export interface ChatRepository {
@@ -15,8 +16,14 @@ export interface ChatRepository {
     conversationId: string,
     options?: GetMessagesOptions,
   ): Promise<ChatMessage[]>;
+
   createMessage(
     conversationId: string,
     message: CreateMessageDTO,
   ): Promise<ChatMessage>;
+
+  updateMessage(
+    messageId: string,
+    updates: UpdateMessageDTO,
+  ): Promise<ChatMessage | null>;
 }