- 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