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/70-poimen.yaml

@@ -27,15 +27,10 @@ metadata:
     app.kubernetes.io/part-of: poimen
   annotations:
     argocd.argoproj.io/sync-wave: "7"
-    # ── Image Updater ─────────────────────────────────────────────────────────
-    argocd-image-updater.argoproj.io/image-list: "workflows=forgejo.riotpiao.com/riotpiao-poimen/poimen-workflows,memory=forgejo.riotpiao.com/riotpiao-poimen/poimen-memory,frontend=forgejo.riotpiao.com/riotpiao-poimen/poimen-frontend"
-    argocd-image-updater.argoproj.io/workflows.update-strategy: digest
-    argocd-image-updater.argoproj.io/workflows.allow-tags: regexp:^latest$
-    argocd-image-updater.argoproj.io/memory.update-strategy: digest
-    argocd-image-updater.argoproj.io/memory.allow-tags: regexp:^latest$
-    argocd-image-updater.argoproj.io/frontend.update-strategy: digest
-    argocd-image-updater.argoproj.io/frontend.allow-tags: regexp:^latest$
-    argocd-image-updater.argoproj.io/write-back-method: argocd
+    # ── Image promotion ──────────────────────────────────────────────────────
+    # Forgejo package webhook → api-gw patches kustomize.images per source.
+    # Source index mapping: 0=workflows, 1=memory, 3=frontend.
+    # Replaced ArgoCD Image Updater (removed 2026-09-10).
 spec:
   project: homelab
   # Multi-source: each repo owns its own k8s manifests.