Add arrstack Vault policies (deployer, KV read, consumer creds)

Grant the Vault access the arrstack engine needs, before any engine
resources exist. Second of three stacked steps (register -> policy ->
resources).

Adds:
- policies/arrstack/admin.yaml: the terraform-vault deployer may
  create/read/update/delete arrstack/config and manage arrstack/roles/*.
- policies/kv/.../arrproxy-admin-token/read.yaml: the deployer may read
  the KV-seeded arrproxy admin token (data + metadata paths) that the
  engine config sources; the existing secret_backends_read policy does
  not cover this kubernetes/namespace KV path.
- policies/arrstack/creds/{sonarr,radarr,prowlarr}.yaml: each terraform-
  <app> run may read its own arrstack/creds/<app> to mint a scoped key.

Policy YAMLs are auto-discovered by policies/policies.hcl, so no wiring
changes are needed.

Apply order: after PR-1 (register). Safe to apply before the engine
exists since these only grant capabilities on paths.

policies/arrstack/admin.yaml

@@ -0,0 +1,27 @@
+# Allow management of the arrstack secrets engine (config and roles) by the
+# terraform-vault deployer.
+---
+rules:
+  - path: "arrstack/config"
+    capabilities:
+      - create
+      - update
+      - read
+      - delete
+  - path: "arrstack/roles/*"
+    capabilities:
+      - create
+      - update
+      - delete
+      - read
+      - list
+  - path: "arrstack/roles"
+    capabilities:
+      - read
+      - list
+
+auth:
+  approle:
+    - tf_vault
+  k8s/au/syd1:
+    - woodpecker_terraform_vault

policies/arrstack/creds/prowlarr.yaml

@@ -0,0 +1,12 @@
+# Allow the terraform-prowlarr run to mint a Prowlarr-scoped arrproxy key.
+---
+rules:
+  - path: "arrstack/creds/prowlarr"
+    capabilities:
+      - read
+
+auth:
+  approle:
+    - terraform_prowlarr
+  k8s/au/syd1:
+    - woodpecker_terraform_prowlarr

policies/arrstack/creds/radarr.yaml

@@ -0,0 +1,12 @@
+# Allow the terraform-radarr run to mint a Radarr-scoped arrproxy key.
+---
+rules:
+  - path: "arrstack/creds/radarr"
+    capabilities:
+      - read
+
+auth:
+  approle:
+    - terraform_radarr
+  k8s/au/syd1:
+    - woodpecker_terraform_radarr

policies/arrstack/creds/sonarr.yaml

@@ -0,0 +1,12 @@
+# Allow the terraform-sonarr run to mint a Sonarr-scoped arrproxy key.
+---
+rules:
+  - path: "arrstack/creds/sonarr"
+    capabilities:
+      - read
+
+auth:
+  approle:
+    - terraform_sonarr
+  k8s/au/syd1:
+    - woodpecker_terraform_sonarr

policies/kv/kubernetes/namespace/arrstack/default/arrproxy-admin-token/read.yaml

@@ -0,0 +1,22 @@
+# Allow the terraform-vault deployer to read the seeded arrproxy admin token so
+# the arrstack engine config module can source it. The token is seeded by
+# argocd-apps #384 at kv/kubernetes/namespace/arrstack/default/arrproxy-admin-token.
+# The deployer's existing secret_backends_read policy only covers
+# kv/data/service/vault/+/+/secret_backend/*, which does not match this
+# kubernetes/namespace path, so this adds the minimal read grant rather than
+# duplicating the secret into the litellm-style path. vault_kv_secret_v2 also
+# reads the kv-v2 metadata path on every plan/apply, so grant that too.
+---
+rules:
+  - path: "kv/data/kubernetes/namespace/arrstack/default/arrproxy-admin-token"
+    capabilities:
+      - read
+  - path: "kv/metadata/kubernetes/namespace/arrstack/default/arrproxy-admin-token"
+    capabilities:
+      - read
+
+auth:
+  approle:
+    - tf_vault
+  k8s/au/syd1:
+    - woodpecker_terraform_vault