feat: webhook info page on / and always show phone status in dashboard

- GET / on port 8081 shows API usage info instead of 404
- Dashboard always shows phone badge (Telefoniert/Frei) alongside AIDA and Teams

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

webhook.py

@@ -13,6 +13,21 @@ logger = logging.getLogger(__name__)
 webhook_app = FastAPI(title="Busylight Webhook")
 
 
+@webhook_app.get("/", response_class=PlainTextResponse)
+async def webhook_info():
+    return """Busylight Webhook API (Telefonanlage)
+
+Endpoints:
+  GET/POST  /{durchwahl}/callstart   - Anruf gestartet (WLED -> Rot)
+  GET/POST  /{durchwahl}/callend     - Anruf beendet (WLED -> zurueck auf Teams-Status)
+
+Beispiel:
+  curl http://localhost:8081/123/callstart
+  curl http://localhost:8081/123/callend
+"""
+
+
+
 def _get_user_by_extension(extension, conn):
     """Find user and their assignment by phone extension."""
     return conn.execute("""