feat: Add Portainer OAuth2 + enable RBAC policy bindings

- Add app-portainer.tf: OAuth2 provider for Portainer container management
- Add portainer_url variable
- Enable RBAC policy bindings for Grafana, ArgoCD, Home Assistant
- Portainer bound to Infrastructure group policy

RBAC Summary:
- Infrastructure group → Grafana, ArgoCD, Portainer
- Home Automation group → Home Assistant
- Media group → arr stack (existing in app-proxy-arr-stack.tf)

app-portainer.tf

@@ -0,0 +1,57 @@
+# =============================================================================
+# Portainer - Container Management
+# =============================================================================
+
+data "authentik_property_mapping_provider_scope" "portainer" {
+  managed_list = [
+    "goauthentik.io/providers/oauth2/scope-openid",
+    "goauthentik.io/providers/oauth2/scope-email",
+    "goauthentik.io/providers/oauth2/scope-profile",
+  ]
+}
+
+resource "authentik_provider_oauth2" "portainer" {
+  name               = "Portainer"
+  client_id          = "portainer"
+  client_type        = "confidential"
+  authorization_flow = data.authentik_flow.default_authorization.id
+  invalidation_flow  = data.authentik_flow.default_invalidation.id
+  
+  access_token_validity  = "hours=1"
+  refresh_token_validity = "days=30"
+  
+  property_mappings = data.authentik_property_mapping_provider_scope.portainer.ids
+  
+  allowed_redirect_uris = [
+    { matching_mode = "strict", url = "${var.portainer_url}/" },
+  ]
+  
+  signing_key = data.authentik_certificate_key_pair.generated.id
+}
+
+resource "authentik_application" "portainer" {
+  name              = "Portainer"
+  slug              = "portainer"
+  protocol_provider = authentik_provider_oauth2.portainer.id
+  
+  meta_description = "Container Management Platform"
+  meta_launch_url  = var.portainer_url
+  
+  group = "Infrastructure"
+}
+
+# Bind Infrastructure group policy to Portainer
+resource "authentik_policy_binding" "portainer_infrastructure" {
+  target = authentik_application.portainer.uuid
+  policy = authentik_policy_expression.infrastructure_access.id
+  order  = 0
+}
+
+output "portainer_client_id" {
+  value = authentik_provider_oauth2.portainer.client_id
+}
+
+output "portainer_client_secret" {
+  value     = authentik_provider_oauth2.portainer.client_secret
+  sensitive = true
+}