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%.

DESIGN.md

@@ -736,6 +736,16 @@ what it was, a separate install, until someone deletes it.
   has no owner-scoped grant below the namespace itself, per §9) — revisit
   if the widened per-tenant-namespace `Secret` access proves too broad in
   practice.
+- A mid-life `spec.teamRef` change on a child CRD (`TerdutEscalationRule`,
+  `TerdutDeadmanSwitch`, `TerdutAlertSource`) isn't specially detected —
+  noticed while grounding Stage 4 against source, not newly introduced by
+  it: all three controllers always resolve `spec.teamRef` fresh every
+  reconcile and act against whatever `TerdutTeam` that currently names,
+  trusting the server-side id already stored in `status` remains valid
+  there. There's no server-side verb that could move an existing
+  integration/policy/switch to a different team in place regardless, so
+  retargeting one onto a live child isn't a supported operation in v1 —
+  delete and recreate the CR instead.
 - Gitops-managed team *membership* (see §4.2).
 - Automatic Deployment restart on upstream Postgres credential rotation.
 - Admission webhooks / CEL-only validation limits (e.g. verifying a