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/repositories/mongo/models/ChatModel.ts

@@ -80,6 +80,10 @@ const ChatMessageSchema = new Schema<
     proposedChange: {
       type: ProposedChangeSchema,
     },
+    respondedAction: {
+      type: String,
+      enum: ["confirm", "reject"],
+    },
   },
   {
     timestamps: true,