Add ephemeral Gitea creds for repospawner (#142)

## Why

`repospawner` creates and seeds Gitea repositories on demand from in-cluster. It needs a Gitea token, and it should not carry a static one -- it gets ephemeral, lease-bound creds like every other service. The `agents` AppRole is CIDR-bound to Ben's workstation, so pods authenticate via Kubernetes auth instead.

## How

- `config/gitea_secret_backend_role/gitea/repospawner.yaml` -- gitea engine role for the `repospawner` user. Scopes `write:repository`, `write:issue`, `read:user` (`read:user` is mandatory: clients validate the login via `GET /api/v1/user`, which 403s without it). ttl 1h / max_ttl 4h. Mirrors `unkin-agent.yaml`.
- `config/auth_kubernetes_role/k8s/au/syd1/repospawner.yaml` -- k8s auth role bound to serviceaccount `repospawner` in namespace `repospawner`, 600s ttl, `audience: vault` (VSO/projected-token flavor, same as `media-apps` / `logging_logarchiver`).
- `policies/gitea/creds/repospawner.yaml` -- `read` on `gitea/creds/repospawner`, bound to `k8s/au/syd1: [repospawner]` only. Deliberately **not** bound to the `agents` AppRole: the service runs in-cluster only.

## Depends on

The terraform-git PR that creates the `repospawner` Gitea user. The engine role cannot mint creds until the user exists -- **merge that one first**.

Reviewed-on: #142
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

config/auth_kubernetes_role/k8s/au/syd1/repospawner.yaml

@@ -0,0 +1,7 @@
+bound_service_account_names:
+  - repospawner
+bound_service_account_namespaces:
+  - repospawner
+token_ttl: 600
+token_max_ttl: 600
+audience: vault

config/gitea_secret_backend_role/gitea/repospawner.yaml

@@ -0,0 +1,19 @@
+# Role minting ephemeral tokens for the repospawner service user. repospawner
+# creates and seeds Gitea repositories on demand, so it gets write on
+# repositories (create + push) and write on issues (issue/PR comments). Read is
+# implied by write. No admin/org/user-write scopes.
+# read:user is required because most API clients validate the login via
+# GET /api/v1/user, which 403s without it.
+# Reading gitea/creds/repospawner mints a lease-bound token deleted from Gitea
+# on revoke/expiry. Consumed by the repospawner pods via Kubernetes auth (see
+# policies/gitea/creds/repospawner.yaml) -- the agents AppRole is CIDR-bound to
+# Ben's workstation and cannot be used from in-cluster.
+---
+username: repospawner
+scopes:
+  - write:repository
+  - write:issue
+  - read:user
+token_name_prefix: vault-repospawner
+ttl: 3600      # 1h
+max_ttl: 14400 # 4h

policies/gitea/creds/repospawner.yaml

@@ -0,0 +1,16 @@
+# Lets the repospawner pods mint ephemeral Gitea tokens for the repospawner
+# service user, so the service authenticates to git.unkin.net as its own
+# least-privilege identity. Reading gitea/creds/repospawner returns a
+# lease-bound token scoped by the role (write:repository, write:issue,
+# read:user -- never merge/admin).
+# Kubernetes auth only: repospawner runs in-cluster and the agents AppRole is
+# CIDR-bound to Ben's workstation.
+---
+rules:
+  - path: "gitea/creds/repospawner"
+    capabilities:
+      - read
+
+auth:
+  k8s/au/syd1:
+    - repospawner