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

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"

app/models/warp_engine/pipeline.rb

@@ -8,8 +8,25 @@ module WarpEngine
 
     belongs_to :software, class_name: "WarpEngine::Software", optional: true
 
+    # Pipelines this record took the software from during the last save, by full name.
+    # The admin says so out loud: a silent reassignment is what made the old behaviour
+    # confusing in the first place.
+    attr_reader :software_taken_from
+
     default_scope { where(deleted_at: nil) }
 
+    # One pipeline per software, and the newest assignment wins.
+    #
+    # `Software#pipeline` is a `has_one`, so two pipelines pointing at the same software
+    # is not an error — it is worse than one: the software keeps showing whichever row
+    # comes first, and assigning it elsewhere looks like it did nothing. Rather than
+    # refusing the assignment, the link moves: whoever held that software lets go of it.
+    #
+    # Deliberately a callback and not a unique index. Rows here are soft-deleted, and a
+    # unique index counts deleted rows too, so a pipeline someone removed last year would
+    # block the software from ever being linked again.
+    before_save :claim_software_from_other_pipelines, if: :will_save_change_to_software_id?
+
     validates :woodpecker_repo_id, presence: true, uniqueness: true
     validates :repo_owner, presence: true
     validates :repo_name, presence: true
@@ -31,6 +48,16 @@ module WarpEngine
       %w[software]
     end
 
+    private
+
+    def claim_software_from_other_pipelines
+      return if software_id.blank?
+
+      others = Pipeline.where(software_id: software_id).where.not(id: id)
+      @software_taken_from = others.map(&:full_name)
+      others.update_all(software_id: nil, updated_at: Time.current)
+    end
+
     ActiveSupport.run_load_hooks(:warp_engine_pipeline, self)
   end
 end