Serialize settings applies; log fallback save error

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/antidriftd/main.go

@@ -82,7 +82,9 @@ func main() {
 	if err := applyFn(cfg); err != nil {
 		log.Printf("settings: %v; falling back to claude backend", err)
 		cfg.AIBackend = "claude"
-		_ = settings.Save(settingsPath, cfg)
+		if err := settings.Save(settingsPath, cfg); err != nil {
+			log.Printf("settings: could not persist claude fallback to %s: %v", settingsPath, err)
+		}
 		if err := applyFn(cfg); err != nil {
 			log.Fatalf("settings: apply failed even with claude: %v", err)
 		}

internal/web/settings_handlers.go

@@ -40,6 +40,8 @@ func (s *Server) handlePostSettings(c *gin.Context) {
 		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid json"})
 		return
 	}
+	s.applyMu.Lock()
+	defer s.applyMu.Unlock()
 	s.settingsMu.Lock()
 	apply := s.applyFn
 	path := s.settingsPath

internal/web/web.go

@@ -35,6 +35,7 @@ type Server struct {
 	settings     settings.Settings
 	settingsPath string
 	applyFn      func(settings.Settings) error
+	applyMu      sync.Mutex // serializes POST /settings apply+save
 }
 
 func NewServer(ctrl *session.Controller) *Server {