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>

spec/models/pipeline_spec.rb

@@ -59,4 +59,42 @@ RSpec.describe WarpEngine::Pipeline do
   describe "associations" do
     it { is_expected.to belong_to(:software).optional }
   end
+
+  # A software has one pipeline. Two pipelines pointing at the same one is not an error
+  # the database catches, it is a link that silently does nothing — so the newest
+  # assignment takes it, and says what it took it from.
+  describe "assigning a software another pipeline already has" do
+    it "moves the link and leaves the other pipeline without one" do
+      software = create(:software)
+      held_by  = create(:pipeline, software: software, repo_owner: "games", repo_name: "old")
+      taking   = create(:pipeline, repo_owner: "games", repo_name: "new")
+
+      taking.update!(software: software)
+
+      expect(taking.reload.software).to eq(software)
+      expect(held_by.reload.software).to be_nil
+      expect(software.reload.pipeline).to eq(taking)
+    end
+
+    it "names the pipelines it took the software from" do
+      software = create(:software)
+      create(:pipeline, software: software, repo_owner: "games", repo_name: "old")
+      taking = create(:pipeline, repo_owner: "games", repo_name: "new")
+
+      taking.update!(software: software)
+
+      expect(taking.software_taken_from).to eq([ "games/old" ])
+    end
+
+    it "leaves other pipelines alone when the software is cleared" do
+      software = create(:software)
+      keeps    = create(:pipeline, software: software, repo_owner: "games", repo_name: "keeps")
+      other    = create(:pipeline, repo_owner: "games", repo_name: "other")
+
+      other.update!(software: nil)
+
+      expect(keeps.reload.software).to eq(software)
+      expect(other.software_taken_from).to be_nil
+    end
+  end
 end