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

.github/copilot-instructions.md

@@ -43,6 +43,7 @@ Use this as your shared context when proposing changes. Keep edits minimal and m
     - `presentation_auto_progress` ("true"/"false", default "true")
     Seeded in `server/init_defaults.py` if missing.
   - Events: Added `page_progress` (Boolean) and `auto_progress` (Boolean) for presentation behavior per event.
+  - WebUntis URL: WebUntis uses the existing Vertretungsplan/Supplement-Table URL (`supplement_table_url`). There is no separate `webuntis_url` setting; use `GET/POST /api/system-settings/supplement-table`.
 
 - Conversions:
   - Enum `ConversionStatus`: `pending`, `processing`, `ready`, `failed`.
@@ -83,6 +84,7 @@ Use this as your shared context when proposing changes. Keep edits minimal and m
     - Detached occurrences (edited/broken out): treated as single events.
   - Single occurrence editing: Users can detach individual occurrences from recurring series. The frontend hooks `actionComplete`/`onActionCompleted` with `requestType='eventChanged'` to persist changes: it calls `POST /api/events/<id>/occurrences/<date>/detach` for single-occurrence edits and `PUT /api/events/<id>` for series or single events as appropriate. The backend creates `EventException` and a standalone `Event` without modifying the master beyond EXDATEs.
   - UI: Events with `SkipHolidays` render a TentTree icon next to the main event icon. The custom recurrence icon in the header was removed; rely on Syncfusion’s native lower-right recurrence badge.
+  - Website & WebUntis: Both event types display a website. WebUntis reads its URL from the system `supplement_table_url` and does not provide a per-event URL field.
 
 - Program info page (`dashboard/src/programminfo.tsx`):
   - Loads data from `dashboard/public/program-info.json` (app name, version, build info, tech stack, changelog).
@@ -153,6 +155,7 @@ Note: Syncfusion usage in the dashboard is already documented above; if a UI for
   - Always compare datetimes in UTC; some DB values may be naive—normalize before comparing (see `routes/events.py`).
   - Scheduler enforces UTC comparisons and normalizes naive timestamps. It publishes only currently active events and clears retained topics for groups with no active events. It also queries a future window (default: 7 days) and expands recurring events using RFC 5545 rules. Event exceptions are respected. Logging is concise and conversion lookups are cached.
 - Use retained MQTT messages for state that clients must recover after reconnect (events per group, client group_id).
+  - Clients should parse `event_type` and then read the corresponding nested payload (`presentation`, `website`, etc.). `website` and `webuntis` use the same nested `website` payload with `type: browser` and a `url`.
 - In-container DB host is `db`; do not use `localhost` inside services.
 - No separate dev vs prod secret conventions: use the same env var keys across environments (e.g., `DB_CONN`, `MQTT_USER`, `MQTT_PASSWORD`).
 - When adding a new route:
@@ -177,6 +180,12 @@ Note: Syncfusion usage in the dashboard is already documented above; if a UI for
 
 ## Scheduler payloads: presentation extras
 - Presentation event payloads now include `page_progress` and `auto_progress` in addition to `slide_interval` and media files. These are sourced from per-event fields in the database (with system defaults applied on event creation).
+ 
+## Scheduler payloads: website & webuntis
+- For both `website` and `webuntis`, the scheduler emits a nested `website` object:
+  - `{ "type": "browser", "url": "https://..." }`
+- The `event_type` remains `website` or `webuntis`. Clients should treat both identically for rendering.
+- The WebUntis URL is set at event creation by reading the system `supplement_table_url`.
 
 Questions or unclear areas? Tell us if you need: exact devcontainer debugging steps, stricter Alembic workflow, or a seed dataset beyond `init_defaults.py`.