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

dashboard/public/program-info.json

@@ -1,6 +1,6 @@
 {
   "appName": "Infoscreen-Management",
-  "version": "2025.1.0-alpha.12",
+  "version": "2025.1.0-alpha.13",
   "copyright": "© 2025 Third-Age-Applications",
   "supportContact": "support@third-age-applications.com",
   "description": "Eine zentrale Verwaltungsoberfläche für digitale Informationsbildschirme.",
@@ -26,10 +26,20 @@
     ]
   },
   "buildInfo": {
-    "buildDate": "2025-10-18T14:00:00Z",
+    "buildDate": "2025-10-19T12:00:00Z",
     "commitId": "9f2ae8b44c3a"
   },
   "changelog": [
+    {
+      "version": "2025.1.0-alpha.13",
+      "date": "2025-10-19",
+      "changes": [
+        "🆕 Events: Neuer Termin-Typ ‘WebUntis’ – nutzt die systemweite Vertretungsplan-URL; Darstellung ident mit ‘Website’.",
+        "🛠️ Scheduler/Clients: Einheitliches Website-Payload für ‘Website’ und ‘WebUntis’ (type: browser, url).",
+        "🛠️ Einstellungen › Events: WebUntis verwendet jetzt die bestehende Vertretungsplan-URL (Supplement-Table); kein separates WebUntis-URL-Feld mehr.",
+        "📖 Doku: MQTT-Event-Payload-Leitfaden und Implementierungsnotizen zu WebUntis/Website ergänzt."
+      ]
+    },
     {
       "version": "2025.1.0-alpha.12",
       "date": "2025-10-18",