fix: code quality, memory safety, and install improvements

Critical fixes:
- Fix Dockerfile: reorder stages so frontend assets embed into Go binary
- Fix Go version 1.25 (nonexistent) to 1.24 across Dockerfile, go.mod, CI
- Add graceful game server shutdown on SIGTERM/SIGINT
- Order startup tasks: updates complete before auto-start
- Fix TOCTOU race in UpdateSettings with atomic Update() method

Security:
- Add optional AUTH_TOKEN bearer auth middleware on API/WS routes
- Fix path traversal in DeleteMod using filepath.Rel instead of HasPrefix
- Add input validation for IPPort, ServerParameters, ScheduledUpdate

Memory safety:
- Cap RPT buffer allocation to 64KB to prevent OOM on large logs
- Cap GetLog file read to 10MB
- Fix context cancel leak in SteamCmdManager.run()
- Remove data-raced cancel field in steamcmd.go
- Atomic file writes (write-temp-then-rename) across all managers

Reliability:
- Log save errors in ProcessManager.Stop()
- Atomic file writes prevent corruption on crash

Tests:
- Add mod_manager_test.go (12 tests: ListWorkshopMods, ListLocalMods,
  BuildUsageMap, RemoveMod, dirSize)
- Add scheduler_test.go (6 tests: Start/Stop, Refresh with empty,
  invalid, valid, and replaced cron expressions)
- Add TestRestart to server_process_test.go

CI/Docs:
- Add -race flag to go test in CI and Makefile
- Add npm lint step to CI
- Add Go/npm module caching to CI
- Update README: prerequisites, AUTH_TOKEN/GIN_MODE/SERVERS_DIR docs,
  fix manual quickstart to use make build

backend/internal/services/steamcmd.go

@@ -25,7 +25,6 @@ type SteamCmdManager struct {
 	serverfileDir string
 	streamer      *LogStreamer
 	running       atomic.Bool
-	cancel        context.CancelFunc
 }
 
 func NewSteamCmdManager(serverfileDir string, streamer *LogStreamer) *SteamCmdManager {
@@ -115,15 +114,13 @@ func (s *SteamCmdManager) run(label string, args []string) error {
 		return fmt.Errorf("start steamcmd: %w", err)
 	}
 
-	s.cancel = cancel
-
 	go s.streamer.Stream(ctx, "steamcmd", stdout, "")
 	go s.streamer.Stream(ctx, "steamcmd", stderr, "")
 
 	go func() {
 		err := cmd.Wait()
+		cancel()
 		s.running.Store(false)
-		s.cancel = nil
 		if err == nil {
 			s.streamer.Broadcast("steamcmd", "[STEAMCMD] SUCCESS: "+label+" finished")
 		} else {