feat: Authentik Terraform configuration for homelab SSO

Infrastructure as Code for Authentik identity provider managing:

OAuth2/OIDC Applications:
- Grafana, Home Assistant, Immich
- Uptime Kuma (proxy auth)
- Sonarr, Radarr, Prowlarr (*arr stack proxy auth)
- ArgoCD

Identity Sources:
- Google Workspace federation

LDAP:
- TrueNAS LDAP provider and outpost

CI/CD:
- GitHub Actions workflow for plan/apply
- Secrets managed via GitHub Actions secrets

Provider: beryju/authentik v2025.2

outputs.tf

@@ -0,0 +1,14 @@
+output "authentik_url" {
+  description = "Authentik instance URL"
+  value       = var.authentik_url
+}
+
+output "admin_group_id" {
+  description = "Admin group ID for RBAC"
+  value       = authentik_group.admins.id
+}
+
+output "users_group_id" {
+  description = "Users group ID"
+  value       = authentik_group.users.id
+}