warp_engine 0.2.0: pluggable storage adapter and publish notifications

Two seams the hosts needed, both backward compatible.

Storage: artifacts are served through WarpEngine::Storage.adapter instead of
raw filesystem calls. The default :local adapter keeps the previous behaviour
byte for byte, including the path traversal guard. A host can now set
config.storage_adapter to any object answering file?/directory?/locate and
serve builds from an object store - FileService and /api/download both honour
a Location.redirect, so a signing adapter turns them into redirects.
DownloadService#create still returns an absolute path (nil when missing) for
existing callers; #locate is the new entry point that can also return a
redirect. Ingestion (upload, extraction, file manager) stays local for now.

Publish: PublishService emits ActiveSupport::Notifications
("warp_engine.publish") with platform/name/version/software/release, so hosts
can react to a new build without hanging callbacks on the models.
WarpEngine.instruments_publish? lets a host feature-detect and keep its
fallback for older engine versions.

app/controllers/warp_engine/api/downloads_controller.rb

@@ -8,23 +8,26 @@ module WarpEngine
     api :GET, "/api/download", "Download a file by path"
     param :path, String, required: true, desc: "File path to download"
     returns code: 200, desc: "File binary data"
+    returns code: 302, desc: "Redirect to the storage location (non-local storage adapter)"
     error code: 400, desc: "Path is blank"
     error code: 404, desc: "File not found"
     def show
       path = params[:path]
       return render(json: { error: "Path is required" }, status: :bad_request) if path.blank?
 
-      full_path = WarpEngine::DownloadService.new.create(
+      location = WarpEngine::DownloadService.new.locate(
         path: path,
         ip: request.remote_ip,
         user_agent: request.user_agent,
         referer: request.referer
       )
 
-      if full_path
-        send_file full_path, disposition: "attachment", type: resolve_mime(full_path)
-      else
+      if location.nil?
         render json: { error: "Not found" }, status: :not_found
+      elsif location.redirect?
+        redirect_to location.url, allow_other_host: true
+      else
+        send_file location.path, disposition: "attachment", type: resolve_mime(location.path)
       end
     end
   end