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/steamcmd.go

@@ -90,7 +90,11 @@ func (s *SteamCmdManager) run(label string, args []string) error {
 
 	s.streamer.Broadcast("steamcmd", "[STEAMCMD] "+label+" starting...")
 
-	cmd := exec.CommandContext(ctx, "steamcmd", args...)
+	steamcmdPath := os.Getenv("STEAMCMD_PATH")
+	if steamcmdPath == "" {
+		steamcmdPath = "steamcmd"
+	}
+	cmd := exec.CommandContext(ctx, steamcmdPath, args...)
 
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {