test(backend): enhance stubs with logging, heartbeat, timed exit, and fake downloads

arma3server_x64 stub:
- Logs binary path and all arguments ([STUB] binary=... args=...)
- Prints heartbeat every 5 seconds
- Accepts -t N flag to exit after N seconds (for auto-exit tests)

steamcmd stub:
- Parses +force_install_dir and +workshop_download_item args
- Creates fake mod directories at the expected workshop content path
- Prints fake download progress and success logs

New tests:
- TestStart_StubLogsArgsAndPath: verifies arg logging via log streamer
- TestStart_StubLogsBinaryPath: verifies binary path logging
- TestStart_StubHeartbeatLogs: verifies heartbeat lines appear within timeout
- TestStart_StubAutoExit: verifies -t flag causes server to auto-exit
- TestDownloadMods_StubCreatesModDirs: verifies single mod dir creation
- TestDownloadMods_StubCreatesMultipleModDirs: verifies multi-mod dir creation
- TestDownloadMods_StubLogsAppear: verifies fake download logs on channel
- TestDownloadMods_StubSteamcmdArgsLogged: verifies install_dir logging

backend/internal/services/steamcmd_test.go

@@ -3,6 +3,7 @@ package services
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 )
@@ -185,3 +186,147 @@ func TestSteamCmdManager_SteampathEnv(t *testing.T) {
 
 	waitForNotRunning(t, sm, 30*time.Second)
 }
+
+func TestDownloadMods_StubCreatesModDirs(t *testing.T) {
+	dir := t.TempDir()
+	streamer := NewLogStreamer()
+
+	sm := NewSteamCmdManager(dir, streamer)
+
+	stubPath, _ := filepath.Abs("testdata/steamcmd")
+	t.Setenv("STEAMCMD_PATH", stubPath)
+
+	modID := "999999"
+	err := sm.DownloadMod(modID)
+	if err != nil {
+		t.Fatalf("DownloadMod error = %v", err)
+	}
+
+	waitForNotRunning(t, sm, 30*time.Second)
+
+	// Stub should have created the workshop mod directory
+	modDir := filepath.Join(dir, "steamapps", "workshop", "content", "107410", modID)
+	fi, err := os.Stat(modDir)
+	if err != nil {
+		t.Fatalf("mod directory should exist after stub download: %v", err)
+	}
+	if !fi.IsDir() {
+		t.Errorf("mod path %q should be a directory", modDir)
+	}
+}
+
+func TestDownloadMods_StubCreatesMultipleModDirs(t *testing.T) {
+	dir := t.TempDir()
+	streamer := NewLogStreamer()
+
+	sm := NewSteamCmdManager(dir, streamer)
+
+	stubPath, _ := filepath.Abs("testdata/steamcmd")
+	t.Setenv("STEAMCMD_PATH", stubPath)
+
+	mods := []string{"111111", "222222", "333333"}
+	err := sm.DownloadMods(mods)
+	if err != nil {
+		t.Fatalf("DownloadMods error = %v", err)
+	}
+
+	waitForNotRunning(t, sm, 30*time.Second)
+
+	for _, modID := range mods {
+		modDir := filepath.Join(dir, "steamapps", "workshop", "content", "107410", modID)
+		if _, err := os.Stat(modDir); os.IsNotExist(err) {
+			t.Errorf("mod directory for %s should exist", modID)
+		}
+	}
+}
+
+func TestDownloadMods_StubLogsAppear(t *testing.T) {
+	dir := t.TempDir()
+	streamer := NewLogStreamer()
+
+	sm := NewSteamCmdManager(dir, streamer)
+
+	stubPath, _ := filepath.Abs("testdata/steamcmd")
+	t.Setenv("STEAMCMD_PATH", stubPath)
+
+	ch := streamer.Subscribe("steamcmd", "test-dl-logs")
+	defer streamer.Unsubscribe("steamcmd", "test-dl-logs")
+
+	err := sm.DownloadMod("444444")
+	if err != nil {
+		t.Fatalf("DownloadMod error = %v", err)
+	}
+
+	// Read lines with generous timeout (stub may take a moment)
+	lines := readSteamcmdLines(ch, 20, 5*time.Second)
+
+	foundDownloading := false
+	foundSuccess := false
+	for _, line := range lines {
+		if strings.Contains(line, "Downloading") && strings.Contains(line, "444444") {
+			foundDownloading = true
+		}
+		if strings.Contains(line, "Success") {
+			foundSuccess = true
+		}
+	}
+	if !foundDownloading {
+		t.Errorf("expected 'Downloading 444444' log line, got %v", lines)
+	}
+	if !foundSuccess {
+		t.Errorf("expected 'Success' log line, got %v", lines)
+	}
+}
+
+func TestDownloadMods_StubSteamcmdArgsLogged(t *testing.T) {
+	dir := t.TempDir()
+	streamer := NewLogStreamer()
+
+	sm := NewSteamCmdManager(dir, streamer)
+
+	stubPath, _ := filepath.Abs("testdata/steamcmd")
+	t.Setenv("STEAMCMD_PATH", stubPath)
+
+	ch := streamer.Subscribe("steamcmd", "test-args-logs")
+	defer streamer.Unsubscribe("steamcmd", "test-args-logs")
+
+	err := sm.DownloadMod("555555")
+	if err != nil {
+		t.Fatalf("DownloadMod error = %v", err)
+	}
+
+	lines := readSteamcmdLines(ch, 20, 5*time.Second)
+
+	foundArgsLog := false
+	for _, line := range lines {
+		if strings.Contains(line, "[STUB]") && strings.Contains(line, "install_dir=") {
+			foundArgsLog = true
+			break
+		}
+	}
+	if !foundArgsLog {
+		t.Errorf("expected [STUB] install_dir= log line, got %v", lines)
+	}
+}
+
+// readSteamcmdLines reads lines from a steamcmd log channel
+func readSteamcmdLines(ch chan string, n int, timeout time.Duration) []string {
+	var lines []string
+	deadline := time.Now().Add(timeout)
+	for len(lines) < n {
+		remaining := time.Until(deadline)
+		if remaining <= 0 {
+			break
+		}
+		select {
+		case line, ok := <-ch:
+			if !ok {
+				return lines
+			}
+			lines = append(lines, line)
+		case <-time.After(remaining):
+			return lines
+		}
+	}
+	return lines
+}