refactor: harden restic configuration

move secrets to .env file so they work on their own, initialize restic repo on fresh deployments

roles/backup/templates/restic-backup.env.j2

@@ -0,0 +1,5 @@
+# Ansible managed
+RESTIC_REPOSITORY="{{ vault_restic_repo_url }}"
+RESTIC_PASSWORD="{{ vault_restic_password }}"
+AWS_ACCESS_KEY_ID="{{ vault_aws_access_key }}"
+AWS_SECRET_ACCESS_KEY="{{ vault_aws_secret_key }}"