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/package.json

@@ -14,7 +14,8 @@
     "dotenv": "^16.4.7",
     "express": "^5.2.1",
     "jsonwebtoken": "^9.0.3",
-    "mongoose": "^9.1.1"
+    "mongoose": "^9.1.1",
+    "rrule": "^2.8.1"
   },
   "devDependencies": {
     "@types/bcrypt": "^6.0.0",