✨(backend) add S3 objects

Inspired by Impress and @sampaccoud's work.

We use Indie Hoster’s Kubernetes objects in staging and production.
In the "dev" environment, we install the `bitnami/minio` chart to mimic
Indie Hoster’s MinIO setup.

To access the MinIO admin interface in dev, use port forwarding;
the interface runs on port 9001.

src/helm/extra/templates/s3.yaml

@@ -0,0 +1,8 @@
+apiVersion: core.libre.sh/v1alpha1
+kind: Bucket
+metadata:
+  name: impress-media-storage
+  namespace: {{ .Release.Namespace | quote }}
+spec:
+  provider: data
+  versioned: true