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/services/ChatService.ts

@@ -47,13 +47,20 @@ export const ChatService = {
   },
 
   getConversations: async (): Promise<ConversationSummary[]> => {
-    throw new Error("Not implemented");
+    return ApiClient.get<ConversationSummary[]>("/chat/conversations");
   },
 
   getConversation: async (
-    _id: string,
-    _options?: GetMessagesOptions,
+    id: string,
+    options?: GetMessagesOptions,
   ): Promise<ChatMessage[]> => {
-    throw new Error("Not implemented");
+    const params = new URLSearchParams();
+    if (options?.before) params.append("before", options.before);
+    if (options?.limit) params.append("limit", options.limit.toString());
+
+    const queryString = params.toString();
+    const url = `/chat/conversations/${id}${queryString ? `?${queryString}` : ""}`;
+
+    return ApiClient.get<ChatMessage[]>(url);
   },
 };