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

scheduler/db_utils.py

@@ -175,6 +175,7 @@ def format_event_with_media(event):
         "start": str(event.start),
         "end": str(event.end),
         "group_id": event.group_id,
+        "event_type": event.event_type.value if event.event_type else None,
         # Carry recurrence metadata for consumers if needed
         "recurrence_rule": getattr(event, "recurrence_rule", None),
         "recurrence_end": (event.recurrence_end.isoformat() if getattr(event, "recurrence_end", None) else None),
@@ -244,6 +245,17 @@ def format_event_with_media(event):
                         f"[Scheduler] Using original file for event_media_id={media.id}: {filename}")
                     _media_decision_logged.add(media.id)
 
-        # Add other event types...
+        # Handle website and webuntis events (both display a website)
+        elif event.event_type.value in ("website", "webuntis"):
+            event_dict["website"] = {
+                "type": "browser",
+                "url": media.url if media.url else None
+            }
+            if media.id not in _media_decision_logged:
+                logging.debug(
+                    f"[Scheduler] Using website URL for event_media_id={media.id} (type={event.event_type.value}): {media.url}")
+                _media_decision_logged.add(media.id)
+
+        # Add other event types (video, message, etc.) here as needed...
 
     return event_dict