feat(helm): add imagePullSecret template for private registry

Changes:
- Add templates/secret.yaml to automatically create docker-registry secret
- Add imageCredentials config to values.yaml (disabled by default)
- Enable imageCredentials in values-production.yaml
- Auto-generates kubernetes.io/dockerconfigjson secret from username/password

Usage in production:
1. Set credentials via ArgoCD values override:
   imageCredentials.username: <from-secret>
   imageCredentials.password: <from-secret>

2. Or use external-secrets operator to inject from vault

The secret will be auto-created and referenced in imagePullSecrets.

helm/peikarband/values-production.yaml

@@ -6,14 +6,16 @@ replicaCount: 1
 image:
   pullPolicy: Always
 
-# Note: If hub-registry-secret doesn't exist, create it with:
-# kubectl create secret docker-registry hub-registry-secret \
-#   --docker-server=hub.peikarband.ir \
-#   --docker-username=<username> \
-#   --docker-password=<password> \
-#   -n peikarband
-# Or disable imagePullSecrets in values.yaml if registry is public
-imagePullSecrets: []
+# Docker registry credentials - automatically creates secret
+imageCredentials:
+  create: true
+  name: hub-registry-secret
+  registry: hub.peikarband.ir
+  username: ""  # TODO: Set via ArgoCD/Helm or external secret
+  password: ""  # TODO: Set via ArgoCD/Helm or external secret
+
+imagePullSecrets:
+  - name: hub-registry-secret
 
 # Reflex configuration for production
 reflex: