feat: add SERVER_BINARY, SERVER_PARAMS, STEAMCMD_PATH env vars

- ProcessManager.Start() reads SERVER_BINARY and SERVER_PARAMS from env
  with platform-based defaults as fallback
- SteamCmdManager.run() reads STEAMCMD_PATH from env, defaults to
  'steamcmd'
- Makefile: add SERVER_BINARY/SERVER_PARAMS/STEAMCMD_PATH variables,
  copy stubs to dev-deploy in 'make dev', pass env vars to backend
- .gitignore: add bin/

backend/internal/services/server_process.go

@@ -85,9 +85,16 @@ func (pm *ProcessManager) Start() error {
 		return fmt.Errorf("save was_running: %w", err)
 	}
 
-	binName := "arma3server_x64"
-	if s.Platform == "windows" {
-		binName = "arma3server_x64.exe"
+	if env := os.Getenv("SERVER_PARAMS"); env != "" {
+		s.ServerParameters = env
+	}
+
+	binName := os.Getenv("SERVER_BINARY")
+	if binName == "" {
+		binName = "arma3server_x64"
+		if s.Platform == "windows" {
+			binName = "arma3server_x64.exe"
+		}
 	}
 	binPath := filepath.Join(pm.serverfileDir, binName)