chore: remove usage tracking and logging functionality

- Deleted the `LoggerPlugin` along with associated usage tracking and in-memory statistics logic.
- Removed all related tests (`logger_plugin_test.go`, `usage_tab_test.go`) and external-facing handler (`usage.go`) for usage statistics export/import.
- Cleaned up TUI integration by deleting `usage_tab.go`.

internal/api/server.go

@@ -31,7 +31,6 @@ import (
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/logging"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/managementasset"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/redisqueue"
-	"github.com/router-for-me/CLIProxyAPI/v6/internal/usage"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/util"
 	sdkaccess "github.com/router-for-me/CLIProxyAPI/v6/sdk/access"
 	"github.com/router-for-me/CLIProxyAPI/v6/sdk/api/handlers"
@@ -507,9 +506,6 @@ func (s *Server) registerManagementRoutes() {
 	mgmt := s.engine.Group("/v0/management")
 	mgmt.Use(s.managementAvailabilityMiddleware(), s.mgmt.Middleware())
 	{
-		mgmt.GET("/usage", s.mgmt.GetUsageStatistics)
-		mgmt.GET("/usage/export", s.mgmt.ExportUsageStatistics)
-		mgmt.POST("/usage/import", s.mgmt.ImportUsageStatistics)
 		mgmt.GET("/config", s.mgmt.GetConfig)
 		mgmt.GET("/config.yaml", s.mgmt.GetConfigYAML)
 		mgmt.PUT("/config.yaml", s.mgmt.PutConfigYAML)
@@ -1001,7 +997,7 @@ func (s *Server) UpdateClients(cfg *config.Config) {
 	}
 
 	if oldCfg == nil || oldCfg.UsageStatisticsEnabled != cfg.UsageStatisticsEnabled {
-		usage.SetStatisticsEnabled(cfg.UsageStatisticsEnabled)
+		redisqueue.SetUsageStatisticsEnabled(cfg.UsageStatisticsEnabled)
 	}
 
 	if s.requestLogger != nil && (oldCfg == nil || oldCfg.ErrorLogsMaxFiles != cfg.ErrorLogsMaxFiles) {