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.

config/providers_oauth2/grafana.yaml

@@ -0,0 +1,17 @@
+# OAuth2/OIDC provider + application for the in-cluster Grafana
+# (grafana.k8s.syd1.au.unkin.net). client_secret is read from Vault, not committed.
+name: Grafana
+authorization_flow: default-provider-authorization-implicit-consent
+invalidation_flow: default-provider-invalidation-flow
+client_type: confidential
+client_id: grafana
+client_secret_vault:
+  mount: kv
+  path: kubernetes/namespace/grafana/default/oauth-credentials
+scope_mappings:
+  - goauthentik.io/providers/oauth2/scope-openid
+  - goauthentik.io/providers/oauth2/scope-email
+  - goauthentik.io/providers/oauth2/scope-profile
+redirect_uris:
+  - matching_mode: strict
+    url: https://grafana.k8s.syd1.au.unkin.net/login/generic_oauth