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/server_process_test.go

@@ -983,3 +983,29 @@ func TestStart_StubWritesRPTLog(t *testing.T) {
 	pm.Stop()
 	time.Sleep(100 * time.Millisecond)
 }
+
+func TestRestart(t *testing.T) {
+	pm, _ := setupStubPM(t, "-t 10")
+
+	if err := pm.Start(); err != nil {
+		t.Fatalf("Start() error = %v", err)
+	}
+	if !pm.IsRunning() {
+		t.Fatal("IsRunning() should be true after Start()")
+	}
+
+	if err := pm.Restart(); err != nil {
+		t.Fatalf("Restart() error = %v", err)
+	}
+	if !pm.IsRunning() {
+		t.Fatal("IsRunning() should be true after Restart()")
+	}
+
+	if err := pm.Stop(); err != nil {
+		t.Fatalf("Stop() error = %v", err)
+	}
+	time.Sleep(100 * time.Millisecond)
+	if pm.IsRunning() {
+		t.Fatal("IsRunning() should be false after Stop()")
+	}
+}