feat: server-level GitHub machine credential for authenticated requests

Anonymous GitHub is capped at 60 requests/hour and cannot read private
repositories, so a machine credential usable by a free (non-enterprise)
account is needed to lift the request budget and reach private release
assets.

- Add internal/githubauth: a process-wide credential delivered via env/secret,
  applied by default to every outbound GitHub request.
- Support two modes: a Personal Access Token sent as `Authorization: Bearer`,
  and a GitHub App that mints a short-lived RS256 JWT (stdlib crypto, no new
  dependency), exchanges it for a ~1h installation token, caches it, and
  single-flights a refresh a few minutes before expiry.
- Inject the credential at the two GitHub call paths: the rpm github provider
  (releases scan + ranged asset-header GETs) and the generic byte proxy
  (private release-asset downloads for github.com hosts).
- Honor precedence: a remote's own username/password overrides the server
  credential; no credential configured stays anonymous.
- Fail closed at startup on partial App configuration; never persist the
  credential to the DB, return it from an API, or log it.
- Read GITHUB_TOKEN / GITHUB_APP_ID / GITHUB_APP_INSTALLATION_ID /
  GITHUB_APP_PRIVATE_KEY[_PATH] via the existing getenv convention.
- Document PAT vs App setup, the free-account fine-grained PAT scopes
  (Contents:read + Metadata:read), precedence, and the rate-limit implication.

internal/provider/generic/generic_test.go

@@ -4,11 +4,56 @@ import (
 	"context"
 	"testing"
 
+	"git.unkin.net/unkin/artifactapi/internal/githubauth"
 	"git.unkin.net/unkin/artifactapi/internal/provider"
 	"git.unkin.net/unkin/artifactapi/internal/provider/generic"
 	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
+type staticCred string
+
+func (s staticCred) Token(context.Context) (string, error) { return string(s), nil }
+
+func TestProvider_AuthHeaders_GitHubServerCredential(t *testing.T) {
+	githubauth.SetServer(staticCred("ghs_server"))
+	t.Cleanup(func() { githubauth.SetServer(nil) })
+
+	p := &generic.Provider{}
+	h, err := p.AuthHeaders(context.Background(), models.Remote{BaseURL: "https://github.com"})
+	if err != nil {
+		t.Fatalf("auth headers: %v", err)
+	}
+	if h.Get("Authorization") != "Bearer ghs_server" {
+		t.Fatalf("Authorization = %q, want Bearer ghs_server", h.Get("Authorization"))
+	}
+}
+
+func TestProvider_AuthHeaders_NonGitHubHostNoServerCredential(t *testing.T) {
+	githubauth.SetServer(staticCred("ghs_server"))
+	t.Cleanup(func() { githubauth.SetServer(nil) })
+
+	p := &generic.Provider{}
+	h, _ := p.AuthHeaders(context.Background(), models.Remote{BaseURL: "https://example.com/downloads"})
+	if h.Get("Authorization") != "" {
+		t.Fatalf("server credential must not be sent to non-github host, got %q", h.Get("Authorization"))
+	}
+}
+
+func TestProvider_AuthHeaders_PerRemoteOverridesServerCredential(t *testing.T) {
+	githubauth.SetServer(staticCred("ghs_server"))
+	t.Cleanup(func() { githubauth.SetServer(nil) })
+
+	p := &generic.Provider{}
+	h, _ := p.AuthHeaders(context.Background(), models.Remote{
+		BaseURL:  "https://github.com",
+		Username: "user",
+		Password: "pass",
+	})
+	if got := h.Get("Authorization"); got != "Basic dXNlcjpwYXNz" {
+		t.Fatalf("per-remote Basic auth must win, got %q", got)
+	}
+}
+
 func TestProvider_Type(t *testing.T) {
 	p := &generic.Provider{}
 	if p.Type() != models.PackageGeneric {