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)

package-lock.json

@@ -67,7 +67,8 @@
         "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",
@@ -13124,6 +13125,15 @@
         "node": ">= 18"
       }
     },
+    "node_modules/rrule": {
+      "version": "2.8.1",
+      "resolved": "https://registry.npmjs.org/rrule/-/rrule-2.8.1.tgz",
+      "integrity": "sha512-hM3dHSBMeaJ0Ktp7W38BJZ7O1zOgaFEsn41PDk+yHoEtfLV+PoJt9E9xAlZiWgf/iqEqionN0ebHFZIDAp+iGw==",
+      "license": "BSD-3-Clause",
+      "dependencies": {
+        "tslib": "^2.4.0"
+      }
+    },
     "node_modules/run-parallel": {
       "version": "1.2.0",
       "resolved": "https://registry.npmjs.org/run-parallel/-/run-parallel-1.2.0.tgz",