Add Ceph dashboard SAML provider

Ceph dashboard SSO is SAML 2.0 (no native OIDC), so onboard it via an Authentik
SAML provider + application. Also resolve SAML authorization/invalidation flows
by slug and the signing keypair by name (mirrors the oauth2 handling), since the
SAML path had not been exercised before.

- config/providers_saml/ceph.yaml: SP entity id/ACS derived from the dashboard
  base URL (audience .../auth/saml2/metadata, acs .../auth/saml2, HTTP-POST),
  signed with the built-in self-signed keypair.

Ceph side (separate, Puppet): ceph dashboard sso setup saml2
  https://dashboard.ceph.unkin.net <authentik-idp-metadata-url>

Validated with `terragrunt plan`: 2 to add (provider + application).

modules/authentik/main.tf

@@ -52,17 +52,34 @@ resource "authentik_property_mapping_provider_scope" "groups_hierarchical" {
   EOT
 }
 
+# Resolve SAML flows by slug and the signing keypair by name, so configs use
+# human-readable names instead of Authentik UUIDs (mirrors the oauth2 handling).
+data "authentik_flow" "saml_authorization" {
+  for_each = var.providers_saml
+  slug     = each.value.authorization_flow
+}
+
+data "authentik_flow" "saml_invalidation" {
+  for_each = var.providers_saml
+  slug     = each.value.invalidation_flow
+}
+
+data "authentik_certificate_key_pair" "saml_signing" {
+  for_each = { for k, v in var.providers_saml : k => v if v.signing_kp != null }
+  name     = each.value.signing_kp
+}
+
 resource "authentik_provider_saml" "this" {
   for_each = var.providers_saml
 
   name               = each.value.name
-  authorization_flow = each.value.authorization_flow
-  invalidation_flow  = each.value.invalidation_flow
+  authorization_flow = data.authentik_flow.saml_authorization[each.key].id
+  invalidation_flow  = data.authentik_flow.saml_invalidation[each.key].id
   acs_url            = each.value.acs_url
   sp_binding         = each.value.sp_binding
   audience           = each.value.audience
   name_id_mapping    = each.value.name_id_mapping
-  signing_kp         = each.value.signing_kp
+  signing_kp         = each.value.signing_kp != null ? data.authentik_certificate_key_pair.saml_signing[each.key].id : null
 }
 
 # Resolve oauth2 flows by slug and scope mappings by managed identifier, and