Native Windows deployment: config file, service, signed auto-update

- internal/config: .env-style config file (gpu-turnstile.env next to the
  exe, -config flag or GPU_TURNSTILE_CONFIG); process env overrides file.
- internal/service: Windows service via golang.org/x/sys/windows/svc —
  graceful SCM stop, 'service install/remove' commands, restart-on-failure
  recovery (also applies staged updates). First external dependency,
  Windows-only; Linux/Docker build unaffected (go.mod stays at 1.23).
- internal/update: polls the Gitea releases API, verifies the Ed25519
  signature of the downloaded binary against an embedded public key
  (openssl-signed by CI), swaps it in next to the running exe, and once
  the GPU lock is idle exits with code 3 so service recovery restarts
  onto the new version. Dev builds and empty pubkey never update.
- CI: tag builds additionally produce gpu-turnstile.exe + .sig + .sha256
  attached to a Gitea release.
- LOG_FILE env var so the service has somewhere to log.

internal/service/service_windows.go

@@ -0,0 +1,120 @@
+//go:build windows
+
+// Package service integrates gpu-turnstile with the Windows Service
+// Control Manager: running as a service with graceful stop, plus
+// install/remove helpers.
+package service
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"time"
+
+	"golang.org/x/sys/windows/svc"
+	"golang.org/x/sys/windows/svc/mgr"
+)
+
+// Name is the Windows service name.
+const Name = "gpu-turnstile"
+
+// IsService reports whether the process is running as a Windows service.
+func IsService() bool {
+	isSvc, err := svc.IsWindowsService()
+	return err == nil && isSvc
+}
+
+// Run executes run as a Windows service. SCM Stop and Shutdown cancel the
+// context passed to run, triggering the same graceful shutdown as SIGTERM
+// in interactive mode.
+func Run(run func(ctx context.Context) error) error {
+	return svc.Run(Name, &handler{run: run})
+}
+
+type handler struct {
+	run func(ctx context.Context) error
+}
+
+func (h *handler) Execute(_ []string, requests <-chan svc.ChangeRequest, status chan<- svc.Status) (bool, uint32) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	status <- svc.Status{State: svc.StartPending}
+	errCh := make(chan error, 1)
+	go func() { errCh <- h.run(ctx) }()
+	status <- svc.Status{State: svc.Running, Accepts: svc.AcceptStop | svc.AcceptShutdown}
+
+	for {
+		select {
+		case err := <-errCh:
+			status <- svc.Status{State: svc.Stopped}
+			if err != nil {
+				return true, 1
+			}
+			return false, 0
+		case c := <-requests:
+			switch c.Cmd {
+			case svc.Interrogate:
+				status <- c.CurrentStatus
+			case svc.Stop, svc.Shutdown:
+				status <- svc.Status{State: svc.StopPending}
+				cancel()
+			}
+		}
+	}
+}
+
+// Install registers gpu-turnstile as an auto-start Windows service whose
+// binPath loads the given config file. Recovery actions restart the
+// service after 5s on failure — this is also what brings up a staged
+// update after the updater exits with a non-zero code.
+func Install(configPath string) error {
+	exe, err := os.Executable()
+	if err != nil {
+		return err
+	}
+	m, err := mgr.Connect()
+	if err != nil {
+		return fmt.Errorf("connect to service manager (run as administrator): %w", err)
+	}
+	defer m.Disconnect()
+
+	binPath := fmt.Sprintf(`"%s" -config "%s"`, exe, configPath)
+	s, err := m.CreateService(Name, binPath, mgr.Config{
+		StartType:   mgr.StartAutomatic,
+		DisplayName: "gpu-turnstile",
+		Description: "GPU arbitration proxy for Ollama and ComfyUI",
+	})
+	if err != nil {
+		return fmt.Errorf("create service: %w", err)
+	}
+	defer s.Close()
+
+	restart := mgr.RecoveryAction{Type: mgr.ServiceRestart, Delay: 5 * time.Second}
+	if err := s.SetRecoveryActions([]mgr.RecoveryAction{restart, restart, restart}, 24*60*60); err != nil {
+		return fmt.Errorf("set recovery actions: %w", err)
+	}
+	if err := s.SetRecoveryActionsOnNonCrashFailures(true); err != nil {
+		return fmt.Errorf("set failure actions flag: %w", err)
+	}
+	return nil
+}
+
+// Remove stops (if running) and unregisters the service.
+func Remove() error {
+	m, err := mgr.Connect()
+	if err != nil {
+		return fmt.Errorf("connect to service manager (run as administrator): %w", err)
+	}
+	defer m.Disconnect()
+	s, err := m.OpenService(Name)
+	if err != nil {
+		return fmt.Errorf("open service: %w", err)
+	}
+	defer s.Close()
+	s.Control(svc.Stop) // ignore error: may already be stopped
+	if err := s.Delete(); err != nil {
+		return fmt.Errorf("delete service: %w", err)
+	}
+	return nil
+}