bao: OIDC login to the browser UI via authelia, as a metadata-only viewer

The bao UI at bao-ui.<swarm> took a raw store token and nothing else.
It now offers an OIDC tab: authelia's `admins` group logs in and lands
on `swarm-operator-viewer`, which is list+read on `secret/metadata/*`
and nothing under `secret/data/` or `sys/`.

- authelia registers an interactive client `swarm-bao-ui`
  (glue-bao-ui-oidc-client.nix) with redirect
  `https://bao-ui.<swarm>/ui/vault/auth/oidc/oidc/callback`; the secret
  publisher carries its secret to
  `secret/swarm/services/swarm-bao-ui/oidc/client`.
- `swarm-bao-granter-role` (bootstrap token) enables the `oidc` auth
  mount with listing visibility `unauth`, asked before attempted like
  cert/approle; `bao-bootstrap-policy.hcl` gains `sys/auth/oidc`.
- The granter's policy gains `auth/oidc/config`, `auth/oidc/role/swarm-*`
  and read on that one secret leaf. It still holds no `sys/auth`.
- New granting unit `swarm-bao-operator-viewer-policy` writes the viewer
  policy, and once the granter may configure `auth/oidc/config` (checked
  through `sys/capabilities-self`), writes the mount's config from the
  published secret and the role binding `groups=admins` to the viewer.
  Before the bootstrap step re-runs it writes the policy, logs the step
  and exits 0.

Route (a) per mara on #4775: enabling the auth method stays a
bootstrap-token step, re-run once on the live store.

module-eval pins the viewer policy's single metadata stanza, that the
granter's policy has no sys/auth path, the oidc enable in the bootstrap
unit, the exit-0 path, the config/role contents, and the client
registration + publish.

nix/host-modules/bao-bootstrap-policy.hcl

@@ -27,6 +27,10 @@ path "sys/auth/approle" {
   capabilities = ["create", "update", "sudo"]
 }
 
+path "sys/auth/oidc" {
+  capabilities = ["create", "update", "sudo"]
+}
+
 # The granter's own policy and role, and nothing it may write.
 path "sys/policies/acl/bao-granter" {
   capabilities = ["create", "update"]