feat: add recurring event deletion with three modes

Implement three deletion modes for recurring events:
- single: exclude specific occurrence via EXDATE mechanism
- future: set RRULE UNTIL to stop future occurrences
- all: delete entire event series

Changes include:
- Add exceptionDates field to CalendarEvent model
- Add RecurringDeleteMode type and DeleteRecurringEventDTO
- EventService.deleteRecurring() with mode-based logic using rrule library
- EventController DELETE endpoint accepts mode/occurrenceDate query params
- recurrenceExpander filters out exception dates during expansion
- AI tools support deleteMode and occurrenceDate for proposed deletions
- ChatService.confirmEvent() handles recurring delete modes
- New DeleteEventModal component for unified delete confirmation UI
- Calendar screen integrates modal for both recurring and non-recurring events

apps/server/src/repositories/mongo/MongoEventRepository.ts

@@ -47,4 +47,17 @@ export class MongoEventRepository implements EventRepository {
     const result = await EventModel.findByIdAndDelete(id);
     return result !== null;
   }
+
+  async addExceptionDate(
+    id: string,
+    date: string,
+  ): Promise<CalendarEvent | null> {
+    const event = await EventModel.findByIdAndUpdate(
+      id,
+      { $addToSet: { exceptionDates: date } },
+      { new: true },
+    );
+    if (!event) return null;
+    return event.toJSON() as unknown as CalendarEvent;
+  }
 }

apps/server/src/repositories/mongo/models/EventModel.ts

@@ -46,6 +46,10 @@ const EventSchema = new Schema<
     recurrenceRule: {
       type: String,
     },
+    exceptionDates: {
+      type: [String],
+      default: [],
+    },
   },
   {
     timestamps: true,