fix(backend): replace mutex with atomic state machine in ProcessManager and SteamCmdManager

- ProcessManager: atomic.Int32 state enum (idle/starting/running/stopping)
  eliminates race between IsRunning check and Start/Stop
- SteamCmdManager: atomic.Bool with CompareAndSwap for same protection
- Restart() now propagates Stop() errors instead of swallowing them

backend/internal/services/steamcmd.go

@@ -6,7 +6,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
-	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -24,9 +24,8 @@ const arma3AppID = "233780"
 type SteamCmdManager struct {
 	serverfileDir string
 	streamer      *LogStreamer
-	mu            sync.Mutex
+	running       atomic.Bool
 	cancel        context.CancelFunc
-	running       bool
 }
 
 func NewSteamCmdManager(serverfileDir string, streamer *LogStreamer) *SteamCmdManager {
@@ -37,18 +36,13 @@ func NewSteamCmdManager(serverfileDir string, streamer *LogStreamer) *SteamCmdMa
 }
 
 func (s *SteamCmdManager) IsRunning() bool {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-	return s.running
+	return s.running.Load()
 }
 
 func (s *SteamCmdManager) UpdateGame(branch, user string) error {
-	s.mu.Lock()
-	if s.running {
-		s.mu.Unlock()
+	if !s.running.CompareAndSwap(false, true) {
 		return fmt.Errorf("steamcmd already running")
 	}
-	s.mu.Unlock()
 
 	args := []string{
 		"+force_install_dir", s.serverfileDir,
@@ -68,12 +62,9 @@ func (s *SteamCmdManager) DownloadMod(modID string) error {
 }
 
 func (s *SteamCmdManager) DownloadMods(modIDs []string) error {
-	s.mu.Lock()
-	if s.running {
-		s.mu.Unlock()
+	if !s.running.CompareAndSwap(false, true) {
 		return fmt.Errorf("steamcmd already running")
 	}
-	s.mu.Unlock()
 
 	if len(modIDs) == 0 {
 		return fmt.Errorf("no mod ids provided")
@@ -104,33 +95,31 @@ func (s *SteamCmdManager) run(label string, args []string) error {
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
 		cancel()
+		s.running.Store(false)
 		return fmt.Errorf("stdout pipe: %w", err)
 	}
 	stderr, err := cmd.StderrPipe()
 	if err != nil {
 		cancel()
+		s.running.Store(false)
 		return fmt.Errorf("stderr pipe: %w", err)
 	}
 
 	if err := cmd.Start(); err != nil {
 		cancel()
+		s.running.Store(false)
 		return fmt.Errorf("start steamcmd: %w", err)
 	}
 
-	s.mu.Lock()
-	s.running = true
 	s.cancel = cancel
-	s.mu.Unlock()
 
 	go s.streamer.Stream("steamcmd", stdout, "")
 	go s.streamer.Stream("steamcmd", stderr, "")
 
 	go func() {
 		err := cmd.Wait()
-		s.mu.Lock()
-		s.running = false
+		s.running.Store(false)
 		s.cancel = nil
-		s.mu.Unlock()
 		if err == nil {
 			s.streamer.Broadcast("steamcmd", "[STEAMCMD] SUCCESS: "+label+" finished")
 		} else {