feat: serve local terraform repos as a provider registry

Local terraform repos already spoke the network mirror protocol, which needs
per-consumer .terraformrc config. This adds the provider registry protocol so
`terraform init` installs from a bare source address
(artifactapi.k8s.../{repo}/{type}) with no client setup.

- serve /.well-known/terraform.json service discovery and the providers.v1
  versions/download endpoints under /terraform/v1/providers
- map the Terraform namespace to the artifactapi repo name and locate the
  provider by type; download_url points back at the existing local file path
- generate SHA256SUMS per version and sign it with a GPG key loaded from
  TF_SIGNING_KEY_PATH; advertise the public key + key id in the download
  response. No key configured -> registry stays disabled (endpoints 404)
- new internal/tfsign (key loading + detached signing) and
  internal/api/terraform (registry handler); export ParseProviderZip for reuse
- add TF_SIGNING_KEY_PATH/PASSPHRASE and TF_PROVIDER_PROTOCOLS config
- unit test signing + verification; dockerised test of the full flow incl.
  signature verification against the advertised key

Also anchor the terraform/ gitignore to the repo root so it stops swallowing
internal/api/terraform and internal/provider/terraform test files (the latter
had gone silently untracked).

internal/api/terraform/registry_test.go

@@ -0,0 +1,186 @@
+package terraform
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+	"golang.org/x/crypto/openpgp"
+	"golang.org/x/crypto/openpgp/armor"
+
+	"git.unkin.net/unkin/artifactapi/internal/database"
+	"git.unkin.net/unkin/artifactapi/internal/testsupport"
+	"git.unkin.net/unkin/artifactapi/internal/tfsign"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+var testDSN string
+
+func TestMain(m *testing.M) {
+	ctx := context.Background()
+	dsn, terminate, err := testsupport.StartPostgres(ctx)
+	if err != nil {
+		os.Exit(m.Run())
+	}
+	testDSN = dsn
+	code := m.Run()
+	terminate()
+	os.Exit(code)
+}
+
+// testSigner writes a throwaway armored key and loads it.
+func testSigner(t *testing.T) *tfsign.Signer {
+	t.Helper()
+	e, err := openpgp.NewEntity("artifactapi test", "tf", "tf@example.com", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var buf bytes.Buffer
+	w, _ := armor.Encode(&buf, openpgp.PrivateKeyType, nil)
+	if err := e.SerializePrivate(w, nil); err != nil {
+		t.Fatal(err)
+	}
+	w.Close()
+	p := filepath.Join(t.TempDir(), "private-key.asc")
+	if err := os.WriteFile(p, buf.Bytes(), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	s, err := tfsign.Load(p, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	return s
+}
+
+func TestProviderRegistryFlow(t *testing.T) {
+	if testDSN == "" {
+		t.Skip("Docker unavailable")
+	}
+	ctx := context.Background()
+	db, err := database.New(testDSN)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	const repo = "tf-reg" // Terraform namespace == repo name
+	const filePath = "unkin/artifactapi/terraform-provider-artifactapi_1.2.3_linux_amd64.zip"
+	const hash = "sha256:983cdb25cb7b976538e4334d26e52dee5f44749b9be1500c760cf5cf66be659b"
+	const wantSha = "983cdb25cb7b976538e4334d26e52dee5f44749b9be1500c760cf5cf66be659b"
+
+	if err := db.CreateRemote(ctx, &models.Remote{Name: repo, PackageType: models.PackageTerraform, RepoType: models.RepoTypeLocal}); err != nil {
+		t.Fatal(err)
+	}
+	if err := db.UpsertBlob(ctx, hash, "blobs/98/3c", 6381007, "application/zip"); err != nil {
+		t.Fatal(err)
+	}
+	if err := db.CreateLocalFile(ctx, repo, filePath, hash); err != nil {
+		t.Fatal(err)
+	}
+
+	signer := testSigner(t)
+	h := NewHandler(db, signer, "5.0,6.0")
+	router := chi.NewRouter()
+	router.Get("/.well-known/terraform.json", h.ServiceDiscovery)
+	router.Mount(MountPath, h.Routes())
+
+	get := func(p string) *httptest.ResponseRecorder {
+		req := httptest.NewRequest("GET", p, nil)
+		w := httptest.NewRecorder()
+		router.ServeHTTP(w, req)
+		return w
+	}
+
+	// Service discovery.
+	w := get("/.well-known/terraform.json")
+	if w.Code != 200 {
+		t.Fatalf("discovery = %d", w.Code)
+	}
+	var disc map[string]string
+	json.Unmarshal(w.Body.Bytes(), &disc)
+	if disc["providers.v1"] != ProvidersV1Path {
+		t.Errorf("providers.v1 = %q", disc["providers.v1"])
+	}
+
+	// Versions.
+	w = get("/terraform/v1/providers/tf-reg/artifactapi/versions")
+	if w.Code != 200 {
+		t.Fatalf("versions = %d %s", w.Code, w.Body)
+	}
+	var vresp struct {
+		Versions []struct {
+			Version   string              `json:"version"`
+			Protocols []string            `json:"protocols"`
+			Platforms []map[string]string `json:"platforms"`
+		} `json:"versions"`
+	}
+	json.Unmarshal(w.Body.Bytes(), &vresp)
+	if len(vresp.Versions) != 1 || vresp.Versions[0].Version != "1.2.3" {
+		t.Fatalf("unexpected versions: %+v", vresp)
+	}
+	if len(vresp.Versions[0].Platforms) != 1 || vresp.Versions[0].Platforms[0]["os"] != "linux" {
+		t.Fatalf("unexpected platforms: %+v", vresp.Versions[0].Platforms)
+	}
+
+	// Download.
+	w = get("/terraform/v1/providers/tf-reg/artifactapi/1.2.3/download/linux/amd64")
+	if w.Code != 200 {
+		t.Fatalf("download = %d %s", w.Code, w.Body)
+	}
+	var dl struct {
+		Filename            string `json:"filename"`
+		DownloadURL         string `json:"download_url"`
+		SHASumsURL          string `json:"shasums_url"`
+		SHASumsSignatureURL string `json:"shasums_signature_url"`
+		SHASum              string `json:"shasum"`
+		SigningKeys         struct {
+			GPGPublicKeys []struct {
+				KeyID      string `json:"key_id"`
+				ASCIIArmor string `json:"ascii_armor"`
+			} `json:"gpg_public_keys"`
+		} `json:"signing_keys"`
+	}
+	json.Unmarshal(w.Body.Bytes(), &dl)
+	if dl.SHASum != wantSha {
+		t.Errorf("shasum = %q", dl.SHASum)
+	}
+	wantURL := "http://example.com/api/v1/local/tf-reg/" + filePath
+	if dl.DownloadURL != wantURL {
+		t.Errorf("download_url = %q, want %q", dl.DownloadURL, wantURL)
+	}
+	if len(dl.SigningKeys.GPGPublicKeys) != 1 || dl.SigningKeys.GPGPublicKeys[0].KeyID != signer.KeyID() {
+		t.Errorf("signing key mismatch: %+v", dl.SigningKeys)
+	}
+
+	// SHA256SUMS + signature verify against the advertised key.
+	sums := get("/terraform/v1/providers/tf-reg/artifactapi/1.2.3/sha256sums")
+	wantLine := wantSha + "  terraform-provider-artifactapi_1.2.3_linux_amd64.zip\n"
+	if sums.Body.String() != wantLine {
+		t.Errorf("sha256sums = %q, want %q", sums.Body.String(), wantLine)
+	}
+	sig := get("/terraform/v1/providers/tf-reg/artifactapi/1.2.3/sha256sums.sig")
+	keyring, err := openpgp.ReadArmoredKeyRing(bytes.NewReader([]byte(dl.SigningKeys.GPGPublicKeys[0].ASCIIArmor)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := openpgp.CheckDetachedSignature(keyring, bytes.NewReader(sums.Body.Bytes()), bytes.NewReader(sig.Body.Bytes())); err != nil {
+		t.Errorf("sha256sums.sig did not verify: %v", err)
+	}
+}
+
+func TestRegistryDisabledWithoutSigner(t *testing.T) {
+	h := NewHandler(nil, nil, "")
+	router := chi.NewRouter()
+	router.Get("/.well-known/terraform.json", h.ServiceDiscovery)
+	req := httptest.NewRequest("GET", "/.well-known/terraform.json", nil)
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+	if w.Code != 404 {
+		t.Errorf("disabled discovery = %d, want 404", w.Code)
+	}
+}