fix(log): ES 7.2.0 time format .SSSZ + index dropdown + demo always visible

- ES query 502 fix: remove +00:00 conversion, use native .SSSZ format
  (ES 7.2.0 date_time = strict_date_optional_time only, rejects +00:00/epoch)
- parseLogTime: use time.RFC3339Nano for fractional seconds
- new API GET /log-datasources/indices to list ES indices via proxy
- LogDatasourceDialog: add '获取索引' button + Select dropdown for ES
- LogDatasourcePage: always show demo sources alongside real ones

backend/router/routes.go

@@ -721,6 +721,7 @@ func Register(r *gin.Engine, db *gorm.DB, log *zap.Logger, cfg *config.Config, c
 	logDSGroup := authorized.Group("/log-datasources")
 	{
 		logDSGroup.GET("", logDSHandler.List)
+		logDSGroup.GET("/indices", logDSHandler.ListIndices)
 		logDSGroup.GET("/:id", logDSHandler.Get)
 		logDSGroup.POST("", logDSHandler.Create)
 		logDSGroup.PUT("/:id", logDSHandler.Update)