A version header on every WarpEngine response, and a movable pipeline link

Every WarpEngine API response now carries `WarpEngine-Version`, so a client can branch on
the engine's age without a round trip to ask. Set in a before_action rather than after:
`rescue_from` never reaches an after_action, and a client needs the version most when
something came back wrong. The name lives in `WarpEngine::VERSION_HEADER`. The host's own
endpoints — the store registry — do not carry it, because they are not the engine.

A software has one pipeline, and the newest assignment now wins. Two pipelines pointing at
the same software was not an error the database caught; it was a link that silently did
nothing, with the software still showing whichever row came first. Assigning a software
another pipeline holds therefore moves it, the admin says which pipeline it was taken
from, and `Pipeline#software_taken_from` carries that for anything else that cares.
Deliberately a callback and not a unique index: rows here are soft-deleted, and a unique
index counts deleted rows, so a pipeline removed last year would block its software from
ever being linked again.

The engine is 0.4.0. The site's /stores page and its screenshot follow the client's new
name, and the shot is a fresh one showing the greyed-out titles the client now lists.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/controllers/warp_engine/api_controller.rb

@@ -5,6 +5,12 @@ module WarpEngine
       formats [ "json" ]
     end
 
+    # Every response the engine serves names the version that served it, so a client can
+    # branch on the engine's age without a round trip to ask. Set *before* the action,
+    # not after: an error handled by `rescue_from` never reaches an after_action, and a
+    # client needs the version most when something came back wrong.
+    before_action :set_version_header
+
     rescue_from StandardError do |e|
       Rails.logger.error("[#{self.class.name}] #{e.class}: #{e.message}")
       render json: { error: "Internal server error" }, status: :internal_server_error
@@ -24,6 +30,10 @@ module WarpEngine
 
     private
 
+    def set_version_header
+      response.headers[WarpEngine::VERSION_HEADER] = WarpEngine::VERSION
+    end
+
     def resolve_mime(path)
       ext = File.extname(path.to_s).delete_prefix(".")
       Mime::Type.lookup_by_extension(ext) || "application/octet-stream"