Fix mediamark Vault secret path and permission group name

The terraform-authentik runner's Vault policy only grants read on
kv/data/kubernetes/namespace/+/default/oauth-credentials (literal trailing
filename), so the arrstack/default/mediamark-oauth-credentials path 403s at
plan time and reddens CI. mediamark deploys in its own `mediamark` namespace
(watchstate model), so point the data source at
kubernetes/namespace/mediamark/default/oauth-credentials, which the policy
covers. Hostnames are unchanged.

Rename the permission group to akP-mediamark-user to match the peer tier-suffix
convention (akP-watchstate-admin, akP-arrstack-user). The group name is derived
from the filename in config/config.hcl, so update the akR-media-adult
reference too.

config/permissions/akP-mediamark-user.yaml

@@ -0,0 +1,4 @@
+# Permission group akP-mediamark-user (name = filename). Grants user access to
+# mediamark: bound to the mediamark application, gating the kids-content
+# marking UI.
+application: mediamark

config/permissions/akP-mediamark.yaml

@@ -1,3 +0,0 @@
-# Permission group akP-mediamark (name = filename). Grants access to mediamark:
-# bound to the mediamark application, gating the kids-content marking UI.
-application: mediamark