feat: serve local terraform repos as a provider registry (#102)

## Why

Local terraform repos already served the Terraform **network mirror** protocol, but consuming that requires every user to add a `provider_installation { network_mirror }` block to `~/.terraformrc`. A `source = "artifactapi.k8s.../ns/type"` address instead triggers the **provider registry** protocol (service discovery at `/.well-known/terraform.json` + GPG-signed SHA256SUMS), which returned 404 — hence *"does not offer a provider registry."*

Local repos are meant to be the real thing, so this makes a terraform local repo a first-class provider registry: `terraform init` installs from a bare source address with no client config.

## What

- Serve `/.well-known/terraform.json` service discovery and the `providers.v1` endpoints under `/terraform/v1/providers`: `versions`, `download/{os}/{arch}`, `sha256sums`, `sha256sums.sig`.
- Map the Terraform **namespace** segment to the artifactapi **repo name**; locate the provider by **type**. `download_url` points back at the existing `/api/v1/local/...` path.
- Generate `SHA256SUMS` per version and sign it with a GPG key loaded from `TF_SIGNING_KEY_PATH` (optional `TF_SIGNING_KEY_PASSPHRASE`); advertise the public key + key id in the download response. **No key → registry stays disabled (endpoints 404)**, so behaviour is unchanged until the signing secret is present.
- New `internal/tfsign` (key load + detached signing, via `x/crypto/openpgp`) and `internal/api/terraform` (registry handler). Export `ParseProviderZip` for reuse.
- `TF_PROVIDER_PROTOCOLS` (default `5.0,6.0`) sets the advertised plugin protocols.
- README section documenting usage.

## Consumer

```hcl
terraform {
  required_providers {
    artifactapi = {
      source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/artifactapi"
      version = "0.1.2"
    }
  }
}
```

## Tests

- `internal/tfsign`: sign + verify round-trip, disabled/missing-key paths.
- `internal/api/terraform`: dockerised full flow (discovery → versions → download → sha256sums → sig), verifying the signature against the advertised public key.

## Follow-ups (separate PRs)

- **argocd-apps**: mount the signing K8s secret into the api deployment + set `TF_SIGNING_KEY_PATH`. The `/` HTTPRoute already routes `/.well-known` and `/terraform` to the API, so no gateway change is needed.
- Image/version bump once tagged.

## Note

Anchored the `terraform/` gitignore to the repo root (`/terraform/`) so it stops matching `internal/*/terraform/`. This surfaced `internal/provider/terraform/terraform_extra_test.go`, which had been silently untracked — now committed.

Reviewed-on: #102
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/testcontainers/testcontainers-go v0.42.0
 	github.com/testcontainers/testcontainers-go/modules/postgres v0.42.0
 	github.com/testcontainers/testcontainers-go/modules/redis v0.42.0
+	golang.org/x/crypto v0.51.0
 	gopkg.in/yaml.v3 v3.0.1
 )
 
@@ -96,7 +97,6 @@ require (
 	go.opentelemetry.io/otel/trace v1.41.0 // indirect
 	go.uber.org/atomic v1.11.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/crypto v0.51.0 // indirect
 	golang.org/x/net v0.53.0 // indirect
 	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/sys v0.44.0 // indirect