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

packages/shared/src/models/ChatMessage.ts

@@ -4,6 +4,8 @@ export type MessageSender = "user" | "assistant";
 
 export type EventAction = "create" | "update" | "delete";
 
+export type RespondedAction = "confirm" | "reject";
+
 export interface ProposedEventChange {
   action: EventAction;
   eventId?: string; // Required for update/delete
@@ -17,6 +19,7 @@ export interface ChatMessage {
   sender: MessageSender;
   content: string;
   proposedChange?: ProposedEventChange;
+  respondedAction?: RespondedAction;
   createdAt?: Date;
 }
 
@@ -43,6 +46,10 @@ export interface GetMessagesOptions {
   limit?: number; // Default: 20
 }
 
+export interface UpdateMessageDTO {
+  respondedAction?: RespondedAction;
+}
+
 export interface ChatResponse {
   message: ChatMessage;
   conversationId: string;