test(backend): add tests for SERVER_BINARY, SERVER_PARAMS, STEAMCMD_PATH env vars

- TestStartWithServerBinaryEnv: custom binary name via env
- TestStartWithServerParamsEnv: overridden params via env
- TestServerParamsEnvOverridesSettings: env takes precedence over settings
- TestSteamCmdManager_SteampathEnv: custom steamcmd path via env

backend/internal/services/steamcmd_test.go

@@ -167,3 +167,21 @@ func TestNewSteamCmdManager(t *testing.T) {
 		t.Error("new manager should not be running")
 	}
 }
+
+func TestSteamCmdManager_SteampathEnv(t *testing.T) {
+	dir := t.TempDir()
+	streamer := NewLogStreamer()
+
+	sm := NewSteamCmdManager(dir, streamer)
+
+	// Point STEAMCMD_PATH at the stub so it doesn't need the real binary
+	stubPath, _ := filepath.Abs("testdata/steamcmd")
+	t.Setenv("STEAMCMD_PATH", stubPath)
+
+	err := sm.DownloadMod("123456")
+	if err != nil {
+		t.Fatalf("DownloadMod with STEAMCMD_PATH env error = %v", err)
+	}
+
+	waitForNotRunning(t, sm, 30*time.Second)
+}