Consolidate Forgejo backup into main Forgejo app

Move backup configuration from separate app to forgejo folder:
- Move forgejo-backup-cronjob.yaml to apps/forgejo/
- Move forgejo-backup-secret-sealed.yaml to apps/forgejo/
- Add backup-namespace.yaml to ensure backup namespace is created
- Remove apps/backup/ folder
- Remove apps/app-backup.yaml Argo CD application

This consolidates the backup configuration into the main Forgejo
application, eliminating the need for a separate Argo CD app.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/app-backup.yaml

@@ -1,17 +0,0 @@
-apiVersion: argoproj.io/v1alpha1
-kind: Application
-metadata:
-  name: backup
-  namespace: argocd
-spec:
-  project: default
-  source:
-    repoURL: git@forgejo.forgejo.svc.cluster.local:basicstack/stack.basicstack.de.git
-    targetRevision: main
-    path: apps/backup
-  destination:
-    server: https://kubernetes.default.svc
-    namespace: backup
-  syncPolicy:
-    syncOptions:
-    - CreateNamespace=true

apps/forgejo/backup-namespace.yaml

@@ -0,0 +1,4 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: backup