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

frontend/src/api/monitor/log-center.ts

@@ -34,6 +34,9 @@ export const logCenterApi = {
   deleteDatasource: (id: number | string) => http.delete(`/log-datasources/${id}`),
   setDefaultDatasource: (id: number | string) => http.post(`/log-datasources/${id}/default`),
   testDatasource: (data: Record<string, unknown>) => http.post('/log-datasources/test', data),
+  /** 通过代理查询 ES 集群索引列表 // Logan.Li */
+  listIndices: (params: Record<string, string>) =>
+    http.get<string[]>('/log-datasources/indices', params),
   queryLoki: (data: {
     datasource_id: number;
     query: string;