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/modlist_manager.go

@@ -103,7 +103,7 @@ func (mm *ModlistManager) Create(name string) (*models.Modlist, error) {
 	if err := os.MkdirAll(mm.dir, 0755); err != nil {
 		return nil, err
 	}
-	if err := os.WriteFile(mm.path(m.ID), data, 0644); err != nil {
+	if err := writeFileAtomic(mm.path(m.ID), data); err != nil {
 		return nil, err
 	}
 	return m, nil
@@ -126,7 +126,7 @@ func (mm *ModlistManager) Update(id, name string, mods []models.ModEntry) (*mode
 	if err != nil {
 		return nil, err
 	}
-	if err := os.WriteFile(mm.path(id), data, 0644); err != nil {
+	if err := writeFileAtomic(mm.path(id), data); err != nil {
 		return nil, err
 	}
 	return m, nil
@@ -160,7 +160,7 @@ func (mm *ModlistManager) Duplicate(id, newName string) (*models.Modlist, error)
 	if err != nil {
 		return nil, err
 	}
-	if err := os.WriteFile(mm.path(dup.ID), data, 0644); err != nil {
+	if err := writeFileAtomic(mm.path(dup.ID), data); err != nil {
 		return nil, err
 	}
 	return dup, nil