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/init_defaults.py

@@ -45,10 +45,10 @@ with engine.connect() as conn:
 
         # Default System Settings anlegen
         default_settings = [
-            ('supplement_table_url', '', 'URL für Vertretungsplan (Stundenplan-Änderungstabelle)'),
+            ('supplement_table_url', '', 'URL für Vertretungsplan / WebUntis (Stundenplan-Änderungstabelle)'),
             ('supplement_table_enabled', 'false', 'Ob Vertretungsplan aktiviert ist'),
             ('presentation_interval', '10', 'Standard Intervall für Präsentationen (Sekunden)'),
-            ('presentation_page_progress', 'true', 'Seitenfortschritt anzeigen (Page-Progress) für Präsentationen'),
+            ('presentation_page_progress', 'true', 'Seitenfortschrift anzeigen (Page-Progress) für Präsentationen'),
             ('presentation_auto_progress', 'true', 'Automatischer Fortschritt (Auto-Progress) für Präsentationen'),
         ]