extend chat model with CRUD actions for event changes

- Add ProposedEventChange type with create/update/delete actions
- Replace proposedEvent with proposedChange in ChatMessage
- Add currentDate to AIContext for time-aware AI responses
- Add AI test endpoint for development (/api/ai/test)
- Fix MongoUserRepository type safety with explicit toUser mapping
- Update CLAUDE.md documentation

CLAUDE.md

@@ -4,7 +4,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 ## Project Overview
 
-**CalChat** is a calendar mobile app with AI support. The core concept is creating calendar events through a chat interface with an AI chatbot (Claude). Users can add, edit, and delete events via natural language conversation.
+**CalChat** is a calendar mobile app with AI support. The core concept is managing calendar events through a chat interface with an AI chatbot. Users can add, edit, and delete events via natural language conversation.
 
 This is a fullstack TypeScript monorepo with npm workspaces.
 
@@ -152,6 +152,7 @@ src/
 - `GET /api/chat/conversations` - Get all conversations (protected)
 - `GET /api/chat/conversations/:id` - Get messages of a conversation with cursor-based pagination (protected)
 - `GET /health` - Health check
+- `POST /api/ai/test` - AI test endpoint (development only)
 
 ### Shared Package (packages/shared)
 
@@ -163,13 +164,15 @@ src/
     ├── User.ts           # User, CreateUserDTO, LoginDTO, AuthResponse
     ├── CalendarEvent.ts  # CalendarEvent, CreateEventDTO, UpdateEventDTO
     └── ChatMessage.ts    # ChatMessage, Conversation, SendMessageDTO, CreateMessageDTO,
-                          # GetMessagesOptions, ChatResponse, ConversationSummary
+                          # GetMessagesOptions, ChatResponse, ConversationSummary,
+                          # ProposedEventChange, EventAction
 ```
 
 **Key Types:**
 - `User`: id, email, displayName, passwordHash?, createdAt?, updatedAt?
 - `CalendarEvent`: id, userId, title, description?, startTime, endTime, note?, isRecurring?, recurrenceRule?
-- `ChatMessage`: id, conversationId, sender ('user' | 'assistant'), content, proposedEvent?
+- `ChatMessage`: id, conversationId, sender ('user' | 'assistant'), content, proposedChange?
+- `ProposedEventChange`: action ('create' | 'update' | 'delete'), eventId?, event?, updates?
 - `Conversation`: id, userId, createdAt?, updatedAt? (messages loaded separately via lazy loading)
 - `CreateEventDTO`: Used for creating events AND for AI-proposed events
 - `GetMessagesOptions`: Cursor-based pagination with `before?: string` and `limit?: number`