add event CRUD actions and recurring event expansion

- Implement full CRUD in MongoEventRepository (findById, findByUserId, findByDateRange, update, delete)
- Extend ChatService to handle create/update/delete actions with dynamic test responses
- Add recurrenceExpander utility using rrule library for RRULE parsing
- Add eventFormatters utility for German-localized week/month overviews
- Add German translations for days and months in shared Constants
- Update client ChatService to support all event actions (action, eventId, updates params)

apps/server/src/controllers/ChatController.ts

@@ -1,5 +1,5 @@
 import { Response } from 'express';
-import { SendMessageDTO, CreateEventDTO } from '@caldav/shared';
+import { SendMessageDTO, CreateEventDTO, UpdateEventDTO, EventAction } from '@caldav/shared';
 import { ChatService } from '../services';
 import { AuthenticatedRequest } from '../middleware';
 
@@ -21,8 +21,21 @@ export class ChatController {
     try {
       const userId = req.user!.userId;
       const { conversationId, messageId } = req.params;
-      const event: CreateEventDTO = req.body;
-      const response = await this.chatService.confirmEvent(userId, conversationId, messageId, event);
+      const { action, event, eventId, updates } = req.body as {
+        action: EventAction;
+        event?: CreateEventDTO;
+        eventId?: string;
+        updates?: UpdateEventDTO;
+      };
+      const response = await this.chatService.confirmEvent(
+        userId,
+        conversationId,
+        messageId,
+        action,
+        event,
+        eventId,
+        updates
+      );
       res.json(response);
     } catch (error) {
       res.status(500).json({ error: 'Failed to confirm event' });