feat(infra): drop in-cluster minio, add pg backup + PVC protection

PROD: object storage moves to UpCloud Managed Object Storage (existing
drops bucket) instead of single-node in-cluster MinIO — durable,
UpCloud-replicated, no PVC to back up.

- Remove forte-drop-minio StatefulSet entirely.
- Add forte-drop-pg-backup CronJob: nightly pg_dump -> gzip -> upload to
  s3://drops/_pgbackups/ (collision-proof prefix), 30-day retention.
  Reuses forte-drop-secrets S3 creds (app user has s3:* on drops).
- PVC prune/delete protection on the postgres volumeClaimTemplate.

infra/overlays/upc-dev/forte-drop-postgresql/resources/postgresql.yaml

@@ -94,6 +94,8 @@ spec:
   volumeClaimTemplates:
   - metadata:
       name: data
+      annotations:
+        argocd.argoproj.io/sync-options: Prune=false,Delete=false
     spec:
       accessModes:
       - ReadWriteOnce