ops(scheduler): add GCS credentials mount for offsite backups

Mount secrets directory and GCS_CREDENTIALS_FILE env var to support
the new gcs_backup_executor in scheduler v1.2.0.

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

stacks/scheduler.yml

@@ -41,6 +41,7 @@ services:
       - DOCS_MIRROR_PATH=/docs-mirror
       - DOCS_CHECK_INTERVAL=21600
       - SCHEDULER_API_KEY=${SCHEDULER_API_KEY}
+      - GCS_CREDENTIALS_FILE=/secrets/gcs-sa-key.json
       - PYTHONPATH=/app
 
     volumes:
@@ -57,6 +58,8 @@ services:
       - /home/jpmschweitzer/.config/code-server:/data/code-server-config:ro
       # For config backups (write destination)
       - /mnt/media/backups/docker-configs:/backups/docker-configs
+      # GCS service account credentials
+      - /home/jpmschweitzer/docker-data/scheduler/secrets:/secrets:ro
 
     networks:
       - docker-dataplane