feat: server-level GitHub machine credential for authenticated requests (#109)

## Why

Anonymous GitHub is capped at 60 requests/hour and cannot read private repositories. A machine credential usable by a free (non-enterprise) account is needed to lift the request budget to ~5000/hr and to read private-repo release assets.

Builds on the background syncer (#108, now merged to `master`); this diff is the auth changes only.

## How

- Add `internal/githubauth`: a process-wide GitHub credential delivered via env/secret, applied by default to every outbound GitHub request (releases scan, ranged asset-header GETs, and the generic-github byte proxy for private assets).
- Support two modes:
  - **PAT** — `GITHUB_TOKEN` sent as `Authorization: Bearer <token>`.
  - **GitHub App** — `GITHUB_APP_ID` + `GITHUB_APP_INSTALLATION_ID` + private key (`GITHUB_APP_PRIVATE_KEY` inline PEM or `GITHUB_APP_PRIVATE_KEY_PATH`). Mint a short-lived RS256 JWT with stdlib `crypto/rsa` (no new dependency), exchange it at `POST /app/installations/{id}/access_tokens` for a ~1h installation token, cache it, and single-flight a refresh a few minutes before expiry.
- Inject at the two GitHub call paths: the rpm github provider header builder (releases + ranged fetches) and the generic provider `AuthHeaders` (byte proxy, github.com hosts only; the pre-signed `objects.githubusercontent.com` redirect deliberately gets no Authorization).
- Honor precedence: a remote's own `username`/`password` overrides the server credential; no credential configured stays anonymous (current behavior).
- Fail closed at startup on partial App configuration (e.g. App id without a private key); a token-and-App conflict is also rejected.
- Never persist the credential to the DB, return it from an API, or log it (token-exchange failures never echo the response body).
- Read config 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.

## Rate limit

Authenticated requests share the syncer's single global limiter — no second limiter is added. A token raises the effective GitHub ceiling (~5000/hr vs ~60/hr), so the limiter defaults stay safe.

## Tests

`internal/githubauth` and `internal/provider/{rpm,generic}`:
- PAT attaches the correct `Authorization` header to releases + asset-header requests.
- App mints a valid RS256 JWT (verified against the app public key), exchanges it at a mocked endpoint, reuses the cached token without re-exchanging, refreshes near expiry, and single-flights concurrent callers.
- Per-remote credential overrides the server credential (rpm + generic).
- No credential → no `Authorization` header, requests still succeed anonymously.
- ETag/304 flow still works with auth attached.
- The credential does not appear in a remote's serialized JSON.
- Config validation: no-config is anonymous; partial App config and token/App conflict both error.

Verified fail-before/pass-after for the injection tests. `gofmt -l`, `go build ./...`, `go vet ./...`, `go test ./...` all clean (26 packages).

Reviewed-on: #109
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/server/server.go

@@ -19,6 +19,7 @@ import (
 	"git.unkin.net/unkin/artifactapi/internal/config"
 	"git.unkin.net/unkin/artifactapi/internal/database"
 	"git.unkin.net/unkin/artifactapi/internal/gc"
+	"git.unkin.net/unkin/artifactapi/internal/githubauth"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/alpine"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/docker"
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/generic"
@@ -66,6 +67,25 @@ func New(cfg *config.Config, version string) (*Server, error) {
 		return nil, fmt.Errorf("s3: %w", err)
 	}
 
+	// Install the process-wide GitHub credential before any provider makes an
+	// outbound call. A misconfiguration (e.g. App id without a private key) fails
+	// closed here rather than silently falling back to anonymous. No credential
+	// configured is fine — requests stay anonymous.
+	ghCred, err := githubauth.New(githubauth.Options{
+		Token:          cfg.GitHubToken,
+		AppID:          cfg.GitHubAppID,
+		InstallationID: cfg.GitHubAppInstallationID,
+		PrivateKeyPEM:  cfg.GitHubAppPrivateKey,
+		PrivateKeyPath: cfg.GitHubAppPrivateKeyPath,
+	})
+	if err != nil {
+		return nil, fmt.Errorf("github auth: %w", err)
+	}
+	githubauth.SetServer(ghCred)
+	if ghCred != nil {
+		slog.Info("github machine credential configured")
+	}
+
 	engine := proxy.NewEngine(db, redis, s3)
 	localHandler := v2.NewLocalHandler(db, s3)
 	virtEngine := virtual.NewEngine(db, engine)