Add S3 backend and storage verification

.env.example

@@ -31,3 +31,24 @@ ARTIFACTSTORE_RETENTION_CONFIG_PATH=
 
 # Default interval for external schedulers that run `artifactstore retention sweep`.
 ARTIFACTSTORE_RETENTION_SWEEP_INTERVAL_SECONDS=3600
+
+# Storage backend selection. `local` is always available; `s3` requires the
+# optional S3 dependency and the S3 settings below.
+ARTIFACTSTORE_STORAGE_BACKENDS=local
+ARTIFACTSTORE_STORAGE_DEFAULT_BACKEND=local
+# Format: producer:retention_class=backend_id, with * wildcards.
+# Example: guide-board:release-evidence=s3,*:*=local
+ARTIFACTSTORE_STORAGE_BACKEND_ROUTES=
+
+# S3-compatible backend settings for Ceph RGW / MinIO / AWS S3.
+ARTIFACTSTORE_S3_ENDPOINT_URL=
+ARTIFACTSTORE_S3_REGION=us-east-1
+ARTIFACTSTORE_S3_BUCKET=
+ARTIFACTSTORE_S3_KEY_PREFIX=artifact-store
+# Secret refs must be env:NAME or file:/mounted/path.
+ARTIFACTSTORE_S3_ACCESS_KEY_REF=
+ARTIFACTSTORE_S3_SECRET_KEY_REF=
+ARTIFACTSTORE_S3_STORAGE_CLASS=
+ARTIFACTSTORE_S3_SSE=
+ARTIFACTSTORE_S3_MULTIPART_THRESHOLD_BYTES=67108864
+ARTIFACTSTORE_S3_MULTIPART_CHUNK_BYTES=8388608