refactor: remove redundant isRecurring property, use recurrenceRule instead

isRecurring was redundant since recurrenceRule as truthy/falsy check suffices.
Removed from shared CalendarEvent type, Mongoose virtual, and all usages.

apps/server/src/utils/recurrenceExpander.ts

@@ -48,7 +48,7 @@ export function expandRecurringEvents(
     // to find events that start before rangeStart but extend into the range
     const adjustedRangeStart = new Date(rangeStart.getTime() - duration);
 
-    if (!event.isRecurring || !event.recurrenceRule) {
+    if (!event.recurrenceRule) {
       // Non-recurring event: add if it overlaps with the range
       if (endTime >= rangeStart && startTime <= rangeEnd) {
         expanded.push({