feat(events): add webuntis event, unify website payload, bump UI to alpha.13

- Add `webuntis` event type; event creation resolves URL from system `supplement_table_url`
- Consolidate settings: remove separate webuntis-url endpoints; use GET/POST /api/system-settings/supplement-table
- Scheduler: emit top-level `event_type` and unified `website` payload (`{ "type":"browser","url":"..." }`) for website/webuntis
- Preserve presentation payloads (page_progress/auto_progress) — presentation messages remain backwards-compatible
- Update defaults (`init_defaults.py`) and remove duplicate webuntis setting
- Docs & metadata: bump program-info to 2025.1.0-alpha.13; update README, copilot-instructions, DEV- and TECH-CHANGELOGs; add MQTT_EVENT_PAYLOAD_GUIDE.md and WEBUNTIS_EVENT_IMPLEMENTATION.md

README.md

@@ -66,6 +66,7 @@ Data flow summary:
 - **Videos**: Media file streaming
 - **Messages**: Text announcements
 - **WebUntis**: Educational schedule integration
+   - Uses the system-wide Vertretungsplan/Supplement-Table URL (`supplement_table_url`) configured under Settings → Events. No separate per-event URL is required; WebUntis events display the same as Website events.
  - **Recurrence & Holidays**: Recurring events can be configured to skip holidays. The backend generates EXDATEs (RecurrenceException) for holiday occurrences using RFC 5545 timestamps (yyyyMMddTHHmmssZ), so the calendar never shows those instances. The scheduler queries a 7-day window to expand recurring events and applies event exceptions, but only publishes events that are active at the current time (UTC). The "Termine an Ferientagen erlauben" toggle does not affect these events.
 - **Single Occurrence Editing**: Users can edit individual occurrences of recurring events without affecting the master series. The system provides a confirmation dialog to choose between editing a single occurrence or the entire series.
 
@@ -196,6 +197,14 @@ For detailed deployment instructions, see:
   - `infoscreen/discovery` - Client registration
   - `infoscreen/{uuid}/heartbeat` - Client alive status
   - `infoscreen/events/{group_id}` - Event distribution
+## 🔗 Scheduler Event Payloads
+
+- Presentations include a `presentation` object with `files`, `slide_interval`, `page_progress`, and `auto_progress`.
+- Website and WebUntis events share a unified payload:
+   - `website`: `{ "type": "browser", "url": "https://..." }`
+- The `event_type` field remains specific (e.g., `presentation`, `website`, `webuntis`) so clients can dispatch appropriately; however, `website` and `webuntis` should be handled identically in clients.
+
+See `MQTT_EVENT_PAYLOAD_GUIDE.md` for details.
   - `infoscreen/{uuid}/group_id` - Client group assignment
 
 ## 📁 Project Structure