authentik: add Grafana OAuth2/OIDC provider + application

Adds an OIDC provider + application so the in-cluster Grafana
(grafana.k8s.syd1.au.unkin.net) can authenticate users against Authentik.

Extends the oauth2 module so provider config stays declarative and
secret-free:
- Resolve authorization/invalidation flows by slug (data.authentik_flow)
  and scope mappings by managed identifier
  (data.authentik_property_mapping_provider_scope).
- Read client_secret from Vault kv-v2 (data.vault_kv_secret_v2) instead of
  committing it; adds the hashicorp/vault provider (auth via VAULT_ADDR/
  VAULT_TOKEN from the Makefile).
- Support allowed_redirect_uris on the oauth2 provider.

config/providers_oauth2/grafana.yaml wires client_id `grafana`, the
openid/email/profile scopes, the login/generic_oauth redirect URI, and
points client_secret at kv/kubernetes/namespace/grafana/default/oauth-credentials.

modules/authentik/versions.tf

@@ -5,5 +5,9 @@ terraform {
       source  = "goauthentik/authentik"
       version = ">= 2026.5.0"
     }
+    vault = {
+      source  = "hashicorp/vault"
+      version = ">= 4.0.0"
+    }
   }
 }