feat(chart): backup CronJob keys S3 off instance.id (code fallback)

templates/backup-cronjob.yaml

@@ -108,8 +108,14 @@ spec:
                   value: {{ .Values.backups.s3.region | quote }}
                 - name: S3_BUCKET
                   value: {{ .Values.backups.s3.bucket | quote }}
+                # S3_PREFIX -- the instance's durable identity. Prefer
+                # instance.id (`<code>-<shortid>`, unique per lifetime,
+                # written by Tower at create + backfill); fall back to
+                # the bare code for overlays not yet carrying an id
+                # (transition window). Once every overlay has an id the
+                # fallback is dead and can be dropped.
                 - name: S3_PREFIX
-                  value: {{ .Values.instance.code | quote }}
+                  value: {{ .Values.instance.id | default .Values.instance.code | quote }}
                 - name: RETAIN
                   value: {{ .Values.backups.retain | quote }}
               command: