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.

packages/shared/src/models/CalendarEvent.ts

@@ -6,7 +6,6 @@ export interface CalendarEvent {
   startTime: Date;
   endTime: Date;
   note?: string;
-  isRecurring?: boolean;
   recurrenceRule?: string;
   exceptionDates?: string[]; // ISO date strings (YYYY-MM-DD) for excluded occurrences
   createdAt?: Date;