Fix Grafana alert provisioning: hardcode email address

Grafana provisioning YAML doesn't support env var substitution.
Replace ${ALERT_EMAIL} with hardcoded admin@trails.cool.

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

infrastructure/grafana/provisioning/alerting/alerts.yml

@@ -82,7 +82,7 @@ contactPoints:
       - uid: email-default
         type: email
         settings:
-          addresses: ${ALERT_EMAIL:-admin@trails.cool}
+          addresses: admin@trails.cool
 
 policies:
   - orgId: 1