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/repositories/mongo/MongoEventRepository.ts

@@ -4,15 +4,22 @@ import { EventModel } from './models';
 
 export class MongoEventRepository implements EventRepository {
   async findById(id: string): Promise<CalendarEvent | null> {
-    throw new Error('Not implemented');
+    const event = await EventModel.findById(id);
+    if (!event) return null;
+    return event.toJSON() as unknown as CalendarEvent;
   }
 
   async findByUserId(userId: string): Promise<CalendarEvent[]> {
-    throw new Error('Not implemented');
+    const events = await EventModel.find({ userId }).sort({ startTime: 1 });
+    return events.map(e => e.toJSON() as unknown as CalendarEvent);
   }
 
   async findByDateRange(userId: string, startDate: Date, endDate: Date): Promise<CalendarEvent[]> {
-    throw new Error('Not implemented');
+    const events = await EventModel.find({
+      userId,
+      startTime: { $gte: startDate, $lte: endDate }
+    }).sort({ startTime: 1 });
+    return events.map(e => e.toJSON() as unknown as CalendarEvent);
   }
 
   async create(userId: string, data: CreateEventDTO): Promise<CalendarEvent> {
@@ -22,10 +29,13 @@ export class MongoEventRepository implements EventRepository {
   }
 
   async update(id: string, data: UpdateEventDTO): Promise<CalendarEvent | null> {
-    throw new Error('Not implemented');
+    const event = await EventModel.findByIdAndUpdate(id, data, { new: true });
+    if (!event) return null;
+    return event.toJSON() as unknown as CalendarEvent;
   }
 
   async delete(id: string): Promise<boolean> {
-    throw new Error('Not implemented');
+    const result = await EventModel.findByIdAndDelete(id);
+    return result !== null;
   }
 }