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/admin/pipelines.rb

@@ -46,11 +46,26 @@ ActiveAdmin.register WarpEngine::Pipeline, as: "Pipeline" do
       f.input :platform, as: :select, collection: WarpEngine::PlatformLink::SUPPORTED_PLATFORMS
       f.input :software_id, as: :select,
               collection: WarpEngine::Software.order(:title).map { |s| [ s.title, s.id ] },
-              include_blank: "- none -"
+              include_blank: "- none -",
+              hint: "One pipeline per software. Picking one that another pipeline already " \
+                    "has moves the link here rather than refusing it."
     end
     f.actions
   end
 
+  controller do
+    # The reassignment itself is the model's job; this only makes it visible. Without a
+    # word about it, the other pipeline loses its software with nothing on screen to say
+    # that it happened.
+    def update
+      super
+      taken = resource.software_taken_from
+      return if taken.blank?
+
+      flash[:notice] = [ flash[:notice], "Software taken from #{taken.join(', ')}." ].compact.join(" ")
+    end
+  end
+
   sidebar "Details", only: :show do
     attributes_table_for resource do
       row :id