gitea: add the gitea token secrets engine (mount, config, teabot roles) (#101)

## Why

teabot's implementer and reviewer bot users should mint **ephemeral, scoped Gitea tokens** on demand rather than holding standing personal access tokens — Gitea tokens never expire on their own, so a leak lasts until someone notices. This registers and mounts the new `vault-plugin-secrets-gitea` engine (released v0.1.0) and declares its roles, mirroring the rancher engine wiring.

## Change

- Register the plugin in the catalog (`config/plugins/vault-plugin-secrets-gitea.yaml`), pinned to the released v0.1.0 binary `sha256 8f67fbc216effada5fd7399888a710b62fad83be0b31761a439e7dec3d56509b` (sha256 of `/opt/openbao-plugins/vault-plugin-secrets-gitea` from the released `openbao-plugin-secrets-gitea-0.1.0` RPM).
- Add `gitea_secret_backend` + `gitea_secret_backend_role` modules and wire them through `config.hcl`, `environments/au/syd1/terragrunt.hcl`, and `modules/vault_cluster` variables/main, using the `giteavaultsecret` provider from the `terraform-unkin` registry (v0.1.0).
- Mount the engine at `gitea/` against `https://git.unkin.net`; seeded site-admin credentials are read from KV (`service/vault/au/syd1/secret_backend/gitea/config`, keys `admin_username`/`admin_password`) — not stored in git.
- **The seed is consumed create-only**: `lifecycle ignore_changes` on `admin_username`/`admin_password` means the engine reads the KV seed only when first creating `gitea/config`. After `rotate-root` diverges the live password from the seed, a later apply never pushes the stale seed back.
- Add roles with conservative, minimal scopes (write: implies read:):
  - `teabot-implementer` — `write:repository`, `write:issue` (clone/push, open PRs, comment).
  - `teabot-reviewer` — `read:repository`, `write:issue` (read diffs, post PR reviews/comments).
- TTLs: `ttl` 1h / `max_ttl` 4h on both roles.

## The site-admin bot + KV seed are now provisioned by Terraform (no manual gap)

Per Ben's review, creating the site-admin bot and seeding its credential is no longer a manual step:

- **terraform-git #46** creates the `gitea-vault-admin` site-admin bot and writes its generated password **once** to `kv/service/vault/au/syd1/secret_backend/gitea/config` (create-only KV write; never updated).
- **terraform-vault #102** grants terraform-git write access to that KV path.

## Ordering (merge + apply)

1. **puppet-prod #498** — installs the plugin binary on the vault nodes (Puppet must run).
2. **terraform-vault #100** (`benvin/gitea-deployer-access`) — deployer access to the gitea mount.
3. **terraform-vault #102** (`benvin/gitea-kv-writer`) — terraform-git KV write grant.
4. **terraform-git #46** (`benvin/gitea-vault-admin`) — creates the bot + seeds KV.
5. **This PR** — mounts the engine (reads the seed) and declares roles.

Files here are disjoint from #100 and #102 (no conflict).

**CI note:** the plan for this PR may hard-fail in CI if the plugin isn't yet registered/installed or the KV seed isn't present in the plan's target. If CI plan fails for that ordering reason, that is expected — do not force; apply only once steps 1–4 are live.

## Remaining manual step (one, ordered)

After this PR's first apply, run `vault write -f gitea/config/rotate-root` so the standing seed password is replaced by one only Vault holds. (On future binary upgrades, bump the RPM version in puppet-prod and the catalog `sha256` here together, then `vault write sys/plugins/reload/backend plugin=vault-plugin-secrets-gitea`.)

https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv
Reviewed-on: #101
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

config/gitea_secret_backend_role/gitea/teabot-implementer.yaml

@@ -0,0 +1,14 @@
+# Role minting ephemeral tokens for the teabot-implementer bot user.
+# The implementer clones/pushes code and opens pull requests, so it gets write
+# on repositories (clone + push + PR create) and write on issues (PR/issue
+# comments). Read is implied by write. No admin/org/user-write scopes.
+# Reading gitea/creds/teabot-implementer mints a lease-bound token deleted from
+# Gitea on revoke/expiry.
+---
+username: teabot-implementer
+scopes:
+  - write:repository
+  - write:issue
+token_name_prefix: vault-teabot-implementer
+ttl: 3600      # 1h
+max_ttl: 14400 # 4h