Manage the agent API service account and token in Terraform

The Authentik API token used by estate automation was created by hand in the
UI and pasted into Vault, so it was undocumented, unauditable and impossible
to rotate reproducibly. Model it as config instead.

Add a service_accounts config kind, discovered from config/service_accounts/
like the other kinds. Each entry creates a service_account user, an RBAC role
carrying its global permissions, its API tokens, and (optionally) a kv-v2
write publishing each token key.

Add sa-agent-api granting view_outpost, view_token and view_token_key, with a
non-expiring api token agent-api-token published to kv/service/authentik/agent-api-token.

README.md

@@ -9,6 +9,7 @@ Terraform configuration for managing the Authentik identity provider at identity
 - **OAuth2/OIDC providers** — OAuth2 and OpenID Connect integrations
 - **LDAP providers** — LDAP provider and outpost configuration
 - **Applications** — application definitions linked to providers
+- **Service accounts** — machine identities with RBAC roles and API tokens (keys published to Vault kv)
 
 ## Configuration
 
@@ -19,7 +20,8 @@ config/
 ├── groups/              # Group definitions
 ├── providers_saml/      # SAML provider definitions
 ├── providers_oauth2/    # OAuth2/OIDC provider definitions
-└── providers_ldap/      # LDAP provider definitions
+├── providers_ldap/      # LDAP provider definitions
+└── service_accounts/    # Automation service accounts + API tokens
 ```
 
 ## Usage