storage: S3 client wrapper and admin Basic-auth middleware

S3 module wraps aws-sdk-s3 against MinIO. Two clients on purpose:

- The bytes-mover client points at S3_ENDPOINT (internal minio:9000 over
  the storage Docker network).
- The presign-only client points at S3_PUBLIC_ENDPOINT (https://s3.asxp.io
  via Traefik) so the URLs it mints resolve from a browser.

force_path_style on both because subdomain-style URLs would require wildcard
DNS under s3.asxp.io.

AdminAuth is a tiny Rack middleware that 401s any request under /admin
unless HTTP Basic credentials match ADMIN_USER / ADMIN_PASSWORD from env.
Fails closed if either env var is missing.

lib/s3.rb

@@ -0,0 +1,50 @@
+require 'aws-sdk-s3'
+
+module S3
+  module_function
+
+  def client
+    @client ||= Aws::S3::Client.new(
+      region:           ENV.fetch('S3_REGION', 'us-east-1'),
+      endpoint:         ENV.fetch('S3_ENDPOINT'),
+      access_key_id:    ENV.fetch('S3_ACCESS_KEY'),
+      secret_access_key: ENV.fetch('S3_SECRET_KEY'),
+      force_path_style: true
+    )
+  end
+
+  # Separate client whose endpoint is the public hostname; used only to mint
+  # presigned URLs that a browser can dereference. Bytes never travel through
+  # this client.
+  def presign_client
+    @presign_client ||= Aws::S3::Client.new(
+      region:           ENV.fetch('S3_REGION', 'us-east-1'),
+      endpoint:         ENV.fetch('S3_PUBLIC_ENDPOINT'),
+      access_key_id:    ENV.fetch('S3_ACCESS_KEY'),
+      secret_access_key: ENV.fetch('S3_SECRET_KEY'),
+      force_path_style: true
+    )
+  end
+
+  def bucket
+    ENV.fetch('S3_BUCKET')
+  end
+
+  def put(key, bytes, content_type: 'application/pdf')
+    client.put_object(
+      bucket:       bucket,
+      key:          key,
+      body:         bytes,
+      content_type: content_type
+    )
+    key
+  end
+
+  def presigned_url(key, expires_in: 300, filename: nil)
+    params = { bucket: bucket, key: key }
+    if filename
+      params[:response_content_disposition] = %(attachment; filename="#{filename}")
+    end
+    Aws::S3::Presigner.new(client: presign_client).presigned_url(:get_object, **params, expires_in: expires_in)
+  end
+end