fix(cron): add Matrix to scheduler delivery platform_map

Matrix is a supported gateway platform but was missing from the
cron scheduler's delivery platform_map, causing cron job results
to silently fail delivery when targeting Matrix rooms.

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

cron/scheduler.py

@@ -136,6 +136,7 @@ def _deliver_result(job: dict, content: str) -> None:
         "slack": Platform.SLACK,
         "whatsapp": Platform.WHATSAPP,
         "signal": Platform.SIGNAL,
+        "matrix": Platform.MATRIX,
         "email": Platform.EMAIL,
         "sms": Platform.SMS,
     }