fix(dev): 🐛 revert automated-upload damage — restore working image pins + OIDC secrets

Automated upload (95deeef) overwrote 5 manually-pinned values:

- forgejo-server: restore workflow-webhook-20260305 (DB has v15a/v15b
  migrations; rolling back to 14.0.2-edp1-rootless WILL break the DB)
- garm: restore v0.1.7-forgejo-22 (v0.1.7-forgejo-23 has exec format
  error — wrong arch build, crashes on OTC CCE amd64 nodes)
- sizer-receiver/secret.yaml: re-add sizer-oidc-client secret (deleted
  by upload; causes OIDC auth failure on every sizer-receiver login)
- dex/manifests/dex-sizer-client.yaml: re-add (deleted by upload;
  dex cannot resolve sizer OIDC client without this secret)
- dex.yaml: restore manifests source block (removed by upload;
  without it ArgoCD never deploys the dex/manifests/ directory)

backup-alerts.yaml (new VMRule from automated upload) is kept as-is.

otc/dev.t09.de/stacks/ci-sizer/sizer-receiver/secret.yaml

@@ -0,0 +1,9 @@
+apiVersion: v1
+kind: Secret
+metadata:
+  name: sizer-oidc-client
+  labels:
+    app: sizer-receiver
+type: Opaque
+stringData:
+  client-secret: "73eda9068bd00dfe67d29f087b5540cb1cd82cc1dd2ac0f838558ac8bbcfcb3a"