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/system_settings.py

@@ -173,7 +173,7 @@ def update_supplement_table_settings():
             url_setting = SystemSetting(
                 key='supplement_table_url',
                 value=url,
-                description='URL für Vertretungsplan (Stundenplan-Änderungstabelle)'
+                description='URL für Vertretungsplan / WebUntis (Stundenplan-Änderungstabelle)'
             )
             session.add(url_setting)
         
@@ -201,3 +201,4 @@ def update_supplement_table_settings():
         return jsonify({'error': str(e)}), 500
     finally:
         session.close()
+