feat(events): reliable holiday skipping for recurrences + UI badge; clean logs

Backend: generate EventException on create/update when skip_holidays or recurrence changes; emit RecurrenceException (EXDATE) with exact occurrence start time (UTC)
API: return master events with RecurrenceRule + RecurrenceException
Frontend: map RecurrenceException → recurrenceException; ensure SkipHolidays instances never render on holidays; place TentTree icon (black) next to main event icon via template
Docs: update README and Copilot instructions for recurrence/holiday behavior
Cleanup: remove dataSource and debug console logs

README.md

@@ -45,6 +45,7 @@ A comprehensive multi-service digital signage solution for educational instituti
 - Event scheduling with academic period support
 - Media management with presentation conversion
 - Holiday calendar integration
+ - Visual indicators: TentTree icon next to the main event icon marks events that skip holidays (icon color: black)
 
 ### 🎯 **Event System**
 - **Presentations**: PowerPoint/LibreOffice → PDF conversion via Gotenberg
@@ -52,6 +53,7 @@ A comprehensive multi-service digital signage solution for educational instituti
 - **Videos**: Media file streaming
 - **Messages**: Text announcements
 - **WebUntis**: Educational schedule integration
+ - **Recurrence & Holidays**: Recurring events can be configured to skip holidays. The backend generates EXDATEs (RecurrenceException) for holiday occurrences so the calendar never shows those instances. The "Termine an Ferientagen erlauben" toggle does not affect these events.
 
 ### 🏫 **Academic Period Management**
 - Support for school years, semesters, and trimesters
@@ -141,6 +143,7 @@ For detailed deployment instructions, see:
 - **Technology**: Flask + SQLAlchemy + Alembic
 - **Database**: MariaDB with timezone-aware timestamps
 - **Features**: RESTful API, file uploads, MQTT integration
+   - Recurrence/holidays: returns only master events with `RecurrenceRule` and `RecurrenceException` (EXDATEs) so clients render recurrences and skip holiday instances reliably.
 - **Port**: 8000
 - **Health Check**: `/health`
 
@@ -271,6 +274,11 @@ mosquitto_sub -h localhost -t "infoscreen/+/heartbeat" -v
 
 ## 🎨 Frontend Features
 
+### Recurrence & holidays
+- The Scheduler (Syncfusion) expands master events via `RecurrenceRule`.
+- The API supplies `recurrenceException` (EXDATE) with exact occurrence start times (UTC) so holiday instances are excluded.
+- Events with “skip holidays” display a TentTree icon next to the main event icon.
+
 ### Syncfusion Components Used (Material 3)
 - **Schedule**: Event calendar with drag-drop support
 - **Grid**: Data tables with filtering and sorting