Add the netbox backend and terraform-infra role (#117)

## Why

- The netbox engine modules stand ready but mount nothing and create no identity until backend and role data exist, so terraform-infra still reads a static NetBox token instead of minting ephemeral scoped tokens.

## How

- Add `config/netbox_secret_backend/netbox.yaml` to mount the engine at `netbox` and point it at the syd1 NetBox URL; the admin token is read from KV, not stored here.
- Add `config/netbox_secret_backend_role/netbox/terraform-infra.yaml` as the single declarative source for the terraform-infra identity: filename-derived role name and NetBox username, write access, short TTLs, and an inline permissions block. Nothing in the file repeats the filename.
- Scope terraform-infra to view/add/change/delete on the IPAM/DCIM objects it manages: prefixes, ip-addresses, ip-ranges, devices, interfaces, mac addresses.
- Add `policies/netbox/creds/terraform-infra.yaml` letting the terraform-infra AppRole and its Woodpecker k8s role read `netbox/creds/terraform-infra`; it attaches to nothing until the separate terraform-infra Vault onboarding lands.

## Dependency order

- Stacked on the modules PR (#115), which stacks on the plugin registration PR. Merge order: plugin -> #115 -> this.

## CI note

- The plan step is red only on the external admin_token KV seed at `kv/data/service/vault/au/syd1/secret_backend/netbox/config` (a NetBox token with add_token + grant_token / superuser). Seeding that path is an environmental prerequisite, not a code defect; everything else validates.

---------

Co-authored-by: BenVincent <benvin@main.unkin.net>
Reviewed-on: #117
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

config/netbox_secret_backend/netbox.yaml

@@ -0,0 +1,16 @@
+# Mounts the netbox token secrets engine at "netbox" and writes its config.
+# The seeded NetBox admin token is sensitive and read from KV, not stored here:
+#   kv/service/vault/au/syd1/secret_backend/netbox/config
+#   -> key: admin_token (required)
+# Populate that KV path with a purpose-built NetBox service token that has
+# add_token + grant_token (or superuser) BEFORE applying, then run
+# `vault write -f netbox/config/rotate` after the first apply so only Vault
+# holds the live admin token.
+#
+# token_version 2 is the NetBox 4.6.5 default and requires API_TOKEN_PEPPERS to
+# be configured on the NetBox server; set token_version: 1 here if the server
+# has no peppers.
+description: "NetBox ephemeral scoped API token engine"
+netbox_url: "https://netbox.k8s.syd1.au.unkin.net"
+token_version: 2
+request_timeout_seconds: 30

config/netbox_secret_backend_role/netbox/terraform-infra.yaml

@@ -0,0 +1,25 @@
+# Single declarative source for the terraform-infra NetBox service identity. The
+# filename stem is the engine role name AND the NetBox username (1:1); config.hcl
+# derives both from it, so neither is repeated below. Creating this file creates
+# the user: the netbox_user_management module synthesizes the NetBox user + object
+# permissions from the permissions block, and the engine role mints ephemeral
+# tokens for that same user. write_enabled true because terraform-infra manages
+# NetBox IPAM/DCIM; very short TTLs because a token is minted per plan/apply and
+# revoked when the run's lease ends.
+---
+write_enabled: true
+ttl: 120     # 2m
+max_ttl: 300 # 5m
+permissions:
+  - object_types:
+      - ipam.prefix
+      - ipam.ipaddress
+      - ipam.iprange
+      - dcim.device
+      - dcim.interface
+      - dcim.macaddress
+    actions:
+      - view
+      - add
+      - change
+      - delete

policies/netbox/creds/terraform-infra.yaml

@@ -0,0 +1,21 @@
+# Allow the terraform-infra runner to mint an ephemeral NetBox token from the
+# terraform-infra role (netbox/creds/terraform-infra), replacing the static
+# netbox_token it used to read from kv/service/terraform/*. The e-breuninger
+# netbox provider authenticates with the minted token; the lease revokes it when
+# the run ends.
+#
+# Bound to both the terraform-infra AppRole and its Woodpecker k8s auth role,
+# mirroring the terraform-ipam pattern. Both principals are created by the
+# terraform-infra Vault onboarding (separate from this netbox change); until
+# that onboarding lands this policy exists but attaches to nothing.
+---
+rules:
+  - path: "netbox/creds/terraform-infra"
+    capabilities:
+      - read
+
+auth:
+  approle:
+    - terraform_infra
+  k8s/au/syd1:
+    - woodpecker_terraform_infra