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

server/routes/events.py

@@ -1,7 +1,7 @@
 from flask import Blueprint, request, jsonify
 from server.permissions import editor_or_higher
 from server.database import Session
-from models.models import Event, EventMedia, MediaType, EventException
+from models.models import Event, EventMedia, MediaType, EventException, SystemSetting
 from datetime import datetime, timezone, timedelta
 from sqlalchemy import and_
 from dateutil.rrule import rrulestr
@@ -375,6 +375,25 @@ def create_event():
         session.commit()
         event_media_id = media.id
 
+    # WebUntis: URL aus System-Einstellungen holen und EventMedia anlegen
+    if event_type == "webuntis":
+        # Hole WebUntis-URL aus Systemeinstellungen (verwendet supplement_table_url)
+        webuntis_setting = session.query(SystemSetting).filter_by(key='supplement_table_url').first()
+        webuntis_url = webuntis_setting.value if webuntis_setting else ''
+        
+        if not webuntis_url:
+            return jsonify({"error": "WebUntis / Supplement table URL not configured in system settings"}), 400
+        
+        # EventMedia für WebUntis anlegen
+        media = EventMedia(
+            media_type=MediaType.website,
+            url=webuntis_url,
+            file_path=webuntis_url
+        )
+        session.add(media)
+        session.commit()
+        event_media_id = media.id
+
     # created_by aus den Daten holen, Default: None
     created_by = data.get("created_by")