Remove MinIO

Garage serves all three buckets and both apps, and a last comparison
found nothing in MinIO that Garage lacks. Drop the MinIO service, its
bootstrap and its root credentials from the deploy, and let compose
remove the leftover containers.

The storage_minio_data volume is left on the host. All images can be
pulled again, so the deploy goes back to failing on a pull error.

Document Garage in the README and SECRETS, including the key format and
how the existing secrets map to it, and swap the MinIO client in the
flake for awscli and rclone.

Assisted-by: Claude:fable-5-2

flake.nix

@@ -1,5 +1,5 @@
 {
-  description = "storage — Postgres + MinIO for asxp.io";
+  description = "storage — Postgres + Garage for asxp.io";
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
@@ -14,7 +14,8 @@
       devShells.${system}.default = pkgs.mkShell {
         buildInputs = [
           pkgs.postgresql_17
-          pkgs.minio-client
+          pkgs.awscli2
+          pkgs.rclone
           pkgs.docker-compose
           pkgs.openssl
         ];
@@ -29,7 +30,7 @@
           echo "storage dev shell ready"
           echo "  docker compose up -d              - bring stack up"
           echo "  psql -h localhost ...             - if you forward 5432"
-          echo "  mc alias set local ...            - point mc at MinIO"
+          echo "  aws --endpoint-url https://s3.asxp.io s3 ...  - talk to Garage"
         '';
       };
     };