feat: remove ArgoCD Image Updater, replaced by Forgejo webhook

Image promotion now handled by api-gw webhook endpoint
(/v1/webhooks/forgejo) which patches ArgoCD Application CRs
on Forgejo package push events (homelab-frontend PR #55).

- Delete Image Updater Application and values
- Remove argocd-image-updater.argoproj.io annotations from all apps
- Registry secrets kept (used by webhook digest queries + imagePullSecrets)

k8s/argocd/apps/55-api-gateway.yaml

@@ -36,11 +36,9 @@ metadata:
     app.kubernetes.io/component: gateway
   annotations:
     argocd.argoproj.io/sync-wave: "7"
-    # ArgoCD Image Updater - auto-update on new image push
-    argocd-image-updater.argoproj.io/image-list: gw=forgejo.riotpiao.com/rock/api-gateway
-    argocd-image-updater.argoproj.io/gw.update-strategy: digest
-    argocd-image-updater.argoproj.io/gw.allow-tags: regexp:^latest$
-    argocd-image-updater.argoproj.io/write-back-method: argocd
+    # Image promotion: Forgejo package webhook → api-gw /v1/webhooks/forgejo
+    # → patches this Application's kustomize.images via k8s API.
+    # Replaced ArgoCD Image Updater (removed 2026-09-10).
 spec:
   project: homelab
   revisionHistoryLimit: 3