Stage 4: TerdutAlertSource

Covers webhook Secret generation/ownership (DESIGN.md §4.5, §7), the
WebhookSecretLost fail-closed condition, and the kind-change
delete-and-recreate rotation path.

Idempotent-create here is deliberately neither adopt-on-409
(Team/service-account) nor list-and-match-by-name (TerdutDeadmanSwitch):
terdut-server shows the webhook key exactly once, at creation, and never
again, so no server-side lookup could ever recover it after a crash.
Instead the generated webhook Secret itself -- written immediately after
the POST, before status is ever touched -- is this CR's only durable
record that a create already succeeded; found with status.integrationID
still unset on a later reconcile, it's read back directly rather than
POSTing a second, orphaned integration. Found missing with
status.integrationID *set* instead, that's the already-designed
WebhookSecretLost case: fail closed, not self-healed, since the key is
genuinely gone and recreating it would rotate a live webhook URL with no
spec change to explain why.

Renaming (PATCH) never touches the key, so it's applied unconditionally
every reconcile, same as the escalation policy's whole-policy PUT. A
spec.kind change is the one case with no in-place update verb at all:
DELETE the old integration, delete the stale webhook Secret, then run the
same create path fresh -- fires a Warning event since this breaks whatever
still sends to the old URL.

Also: fakeTerdutServer grows POST/PATCH/DELETE .../integrations routes
behind a new handleIntegrationSubPath, split out of handleTeamSubPath to
stay under gocyclo's threshold; three goconst-flagged test literals
("does-not-exist", "unready") and one unparam-flagged test helper
parameter (bootstrapReadyTerdutServer's always-"default" namespace) get
shared/removed now that a fourth same-shaped caller made the repetition
concrete enough for the linter to flag.

DESIGN.md §13 gains one honest gap found while grounding this stage, not
introduced by it: no child CRD specially detects a mid-life teamRef
change; all three always resolve spec.teamRef fresh and trust the
already-stored server-side id remains valid there.

make fmt lint test build all clean; internal/controller envtest coverage
holds at 71.6%.

internal/controller/terdutdeadmanswitch_controller_test.go

@@ -32,7 +32,7 @@ var _ = Describe("TerdutDeadmanSwitch Controller", func() {
 		fake, fakeSrv = newFakeTerdutServer()
 		DeferCleanup(fakeSrv.Close)
 
-		srv = bootstrapReadyTerdutServer(ctx, operatorNamespace, uniqueName("dmserver"), fakeSrv.URL)
+		srv = bootstrapReadyTerdutServer(ctx, uniqueName("dmserver"), fakeSrv.URL)
 		team = readyTerdutTeam(ctx, operatorNamespace, uniqueName("dmteam"), srv, fakeSrv.URL)
 
 		reconciler = &TerdutDeadmanSwitchReconciler{
@@ -177,7 +177,7 @@ var _ = Describe("TerdutDeadmanSwitch Controller", func() {
 				ObjectMeta: metav1.ObjectMeta{Name: unreadyName, Namespace: operatorNamespace},
 				Spec: terdutv1alpha1.TerdutTeamSpec{
 					ServerRef:   terdutv1alpha1.TerdutServerRef{Name: srv.Name},
-					DisplayName: "unready",
+					DisplayName: testUnreadyDisplayName,
 				},
 			}
 			Expect(k8sClient.Create(ctx, unready)).To(Succeed())