bao: let the UI's admins login write the review classifier's API key

The docs tell the operator to write
`secret/swarm/services/review-classifier/anthropic-api-key` by hand in
the bao UI, but the UI's OIDC login for authelia's `admins` group lands
on `swarm-operator-viewer`, which is list+read on `secret/metadata/*`
and nothing under `secret/data/`. bao refuses the write, so an admin
can't set the key and every classifier run fails with "no API key".

The viewer policy gains `create` and `update` on that one leaf, no
`read`: a UI session still never sees a secret value. The leaf is a
named binding, shared with the classifier's reader policy.

module-eval pins the viewer's two stanzas and that no `data/` grant
carries `read`. docs/swarm/secrets.md and services.md describe the
write.

Closes #5148

docs/swarm/secrets.md

@@ -524,9 +524,12 @@ stream passthrough readers use on that host comes up with the store sealed.
 
 The OIDC tab logs in through authelia as the client `swarm-bao-ui`
 (`swarm.bao.ui.oidc.clientId`). An `admins` member gets a token under
-`swarm-operator-viewer`: `list` and `read` on `secret/metadata/*`. That shows
+`swarm-operator-viewer`: `list` and `read` on `secret/metadata/*`, and
+`create` and `update` on
+`secret/data/swarm/services/review-classifier/anthropic-api-key`. That shows
 the key tree and each key's versions and timestamps, and no value: the store
-refuses every `secret/data/` read. Anything more needs a token.
+refuses every `secret/data/` read. The one write is the review classifier's API
+key, which the operator fills in by hand. Anything more needs a token.
 
 | piece                                                                               | written by                                                                                       |
 | ----------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------ |

docs/swarm/services.md

@@ -243,7 +243,8 @@ identity: the host that runs the store has it, and a services host with the
 store elsewhere gets the classifier only once it holds a copy of the pair (see
 below). Set `deploy.review-classifier.enable` directly to override either way.
 
-The operator writes the Anthropic key by hand, in the secret store's UI at
+The operator writes the Anthropic key by hand, in the secret store's UI under
+the OIDC login, at
 `secret/swarm/services/review-classifier/anthropic-api-key`, field `api_key`.
 Nothing writes it for you: until it exists the service runs and every run fails
 with `no API key…`. The service reads the key on every run, so writing or

nix/host-modules/swarm-bao.nix

@@ -577,13 +577,18 @@ let
   # classifier's reviews with it; no other principal's policy reaches it.
   classifierForgeTokenLeaf = "swarm/services/review-classifier/forge/token";
 
+  # The review classifier's Anthropic API key. The operator writes it through
+  # the UI's OIDC login; the classifier reads it on every run.
+  classifierApiKeyLeaf = "swarm/services/review-classifier/anthropic-api-key";
+
   # The UI's OIDC client secret. The publisher writes it; the granter reads it
   # into the `oidc` auth method's config, which is the only copy bao uses.
   baoUiClientLeaf = "swarm/services/${cfg.ui.oidc.clientId}/oidc/client";
 
   # What an OIDC login through the UI gets: the key tree and each key's KV
-  # metadata, never a value — KV v2 serves values under `data/`, which no
-  # stanza here names. The UI needs no `sys/` grant on top:
+  # metadata, and a write on the one leaf the operator fills by hand. Never a
+  # value: KV v2 serves values under `data/`, and the one `data/` stanza has
+  # no `read`. The UI needs no `sys/` grant on top:
   # `sys/internal/ui/mounts` lists any mount the token holds a capability
   # under, and the rest it calls is in the `default` policy.
   operatorViewerPolicyName = "swarm-operator-viewer";
@@ -591,6 +596,9 @@ let
     path "${credentialMountPath}/metadata/*" {
       capabilities = ["list", "read"]
     }
+    path "${credentialMountPath}/data/${classifierApiKeyLeaf}" {
+      capabilities = ["create", "update"]
+    }
   '';
 
   # authelia's operator group (`operatorGroup` in ./swarm-authelia.nix, the
@@ -967,7 +975,7 @@ let
       name = "swarm-review-classifier";
       cn = baoDeploy.reviewClassifierCommonName;
       policyText =
-        readStanza "${credentialMountPath}/data/swarm/services/review-classifier/anthropic-api-key"
+        readStanza "${credentialMountPath}/data/${classifierApiKeyLeaf}"
         + readStanza "${credentialMountPath}/data/swarm/services/${hyperhiveCfg.swarm.review-classifier.clientId}/oidc/client";
     }
   ];

nix/module-eval/bao-grants.nix

@@ -1836,10 +1836,11 @@ let
 
     # ── the UI's OIDC login ─────────────────────────────────────────────────
     {
-      # What an `admins` login through the UI holds: the key tree and KV
-      # metadata. Any `data/` path would hand a browser session every secret
-      # in the store, and any `sys/` one more than the UI needs.
-      name = "the operator viewer policy is list and read on KV metadata, and nothing under data/ or sys/";
+      # What an `admins` login through the UI holds: the key tree, KV
+      # metadata, and a write on the one leaf the operator fills by hand.
+      # A `read` under `data/` would hand a browser session a secret's
+      # value, and any `sys/` path more than the UI needs.
+      name = "the operator viewer policy is list and read on KV metadata, a write on the classifier's API key, and no data/ read or sys/";
       ok =
         let
           viewer = grantsIn baoGrantHere.systemd.services.swarm-bao-operator-viewer-policy.script;
@@ -1852,10 +1853,18 @@ let
               "read"
             ];
           }
+          {
+            path = "secret/data/swarm/services/review-classifier/anthropic-api-key";
+            caps = [
+              "create"
+              "update"
+            ];
+          }
         ]
         && grantFor viewer "secret/data/swarm/agents/x/queue" == null
         && grantFor viewer "secret/metadata/swarm/agents/x/queue" != null
-        && !(lib.any (g: lib.hasPrefix "secret/data" g.path || lib.hasPrefix "sys/" g.path) viewer);
+        && !(lib.any (g: lib.hasPrefix "secret/data" g.path && lib.elem "read" g.caps) viewer)
+        && !(lib.any (g: lib.hasPrefix "sys/" g.path) viewer);
     }
     {
       # Route (a): enabling an auth method stays a bootstrap-token act. The