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/utils/recurrenceExpander.ts

@@ -0,0 +1,109 @@
+import { RRule, rrulestr } from 'rrule';
+import { CalendarEvent } from '@caldav/shared';
+
+export interface ExpandedEvent extends CalendarEvent {
+  occurrenceStart: Date;
+  occurrenceEnd: Date;
+}
+
+// Convert local time to "fake UTC" for rrule
+// rrule interprets all dates as UTC internally, so we need to trick it
+function toRRuleDate(date: Date): Date {
+  return new Date(Date.UTC(
+    date.getFullYear(),
+    date.getMonth(),
+    date.getDate(),
+    date.getHours(),
+    date.getMinutes(),
+    date.getSeconds()
+  ));
+}
+
+// Convert rrule result back to local time
+function fromRRuleDate(date: Date): Date {
+  return new Date(
+    date.getUTCFullYear(),
+    date.getUTCMonth(),
+    date.getUTCDate(),
+    date.getUTCHours(),
+    date.getUTCMinutes(),
+    date.getUTCSeconds()
+  );
+}
+
+/**
+ * Expand recurring events into individual occurrences within a date range.
+ * Non-recurring events are returned as-is with occurrenceStart/End = startTime/endTime.
+ */
+export function expandRecurringEvents(
+  events: CalendarEvent[],
+  rangeStart: Date,
+  rangeEnd: Date
+): ExpandedEvent[] {
+  const expanded: ExpandedEvent[] = [];
+
+  for (const event of events) {
+    const startTime = new Date(event.startTime);
+    const endTime = new Date(event.endTime);
+    const duration = endTime.getTime() - startTime.getTime();
+
+    if (!event.isRecurring || !event.recurrenceRule) {
+      // Non-recurring event: add as-is if within range
+      if (startTime >= rangeStart && startTime <= rangeEnd) {
+        expanded.push({
+          ...event,
+          occurrenceStart: startTime,
+          occurrenceEnd: endTime,
+        });
+      }
+      continue;
+    }
+
+    // Recurring event: parse RRULE and expand
+    try {
+      const rule = rrulestr(`DTSTART:${formatRRuleDateString(startTime)}\nRRULE:${event.recurrenceRule}`);
+
+      // Get occurrences within the range (using fake UTC dates)
+      const occurrences = rule.between(
+        toRRuleDate(rangeStart),
+        toRRuleDate(rangeEnd),
+        true // inclusive
+      );
+
+      for (const occurrence of occurrences) {
+        const occurrenceStart = fromRRuleDate(occurrence);
+        const occurrenceEnd = new Date(occurrenceStart.getTime() + duration);
+
+        expanded.push({
+          ...event,
+          occurrenceStart,
+          occurrenceEnd,
+        });
+      }
+    } catch (error) {
+      // If RRULE parsing fails, include the event as a single occurrence
+      console.error(`Failed to parse recurrence rule for event ${event.id}:`, error);
+      expanded.push({
+        ...event,
+        occurrenceStart: startTime,
+        occurrenceEnd: endTime,
+      });
+    }
+  }
+
+  // Sort by occurrence start time
+  expanded.sort((a, b) => a.occurrenceStart.getTime() - b.occurrenceStart.getTime());
+
+  return expanded;
+}
+
+// Format date as RRULE DTSTART string (YYYYMMDDTHHMMSS)
+function formatRRuleDateString(date: Date): string {
+  const year = date.getFullYear();
+  const month = String(date.getMonth() + 1).padStart(2, '0');
+  const day = String(date.getDate()).padStart(2, '0');
+  const hours = String(date.getHours()).padStart(2, '0');
+  const minutes = String(date.getMinutes()).padStart(2, '0');
+  const seconds = String(date.getSeconds()).padStart(2, '0');
+  return `${year}${month}${day}T${hours}${minutes}${seconds}`;
+}