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/api/mods.go

@@ -49,6 +49,14 @@ func (h *Handler) ListMods(c *gin.Context) {
 	c.JSON(http.StatusOK, all)
 }
 
+func isUnderDir(path, dir string) bool {
+	rel, err := filepath.Rel(dir, path)
+	if err != nil {
+		return false
+	}
+	return rel != ".." && !strings.HasPrefix(rel, ".."+string(filepath.Separator))
+}
+
 func (h *Handler) DeleteMod(c *gin.Context) {
 	var input deleteModInput
 	if err := c.ShouldBindJSON(&input); err != nil {
@@ -62,7 +70,7 @@ func (h *Handler) DeleteMod(c *gin.Context) {
 		return
 	}
 
-	if !strings.HasPrefix(absPath, h.serverfileDir) && !strings.HasPrefix(absPath, h.modsDir) {
+	if !isUnderDir(absPath, h.serverfileDir) && !isUnderDir(absPath, h.modsDir) {
 		c.JSON(http.StatusForbidden, gin.H{"error": "path outside allowed directories"})
 		return
 	}