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/controller/log_proxy.go

@@ -149,10 +149,7 @@ func (h *LogProxyHandler) QueryES(c *gin.Context) {
 			},
 		}
 		if req.Start != "" || req.End != "" {
-			// ES 7.2 date_time 格式要求 +00:00 而非 Z；兼容 @timestamp 与 timestamp // Logan.Li
-			buildRangeVal := func(v string) string {
-				return strings.Replace(v, "Z", "+00:00", 1)
-			}
+			// ES 7.2 date_time 仅接受 .SSSZ 格式；兼容 @timestamp 与 timestamp // Logan.Li
 			rangeCond := map[string]interface{}{
 				"bool": map[string]interface{}{
 					"should": []interface{}{
@@ -167,10 +164,10 @@ func (h *LogProxyHandler) QueryES(c *gin.Context) {
 				for _, v := range rng {
 					r := v.(map[string]string)
 					if req.Start != "" {
-						r["gte"] = buildRangeVal(req.Start)
+						r["gte"] = req.Start
 					}
 					if req.End != "" {
-						r["lte"] = buildRangeVal(req.End)
+						r["lte"] = req.End
 					}
 				}
 			}
@@ -217,9 +214,8 @@ func parseLogTime(s string) (time.Time, error) {
 		}
 		return time.Unix(n, 0), nil
 	}
-	// ES 7.2 date_time 不接受 Z，转 +00:00
-	s = strings.Replace(s, "Z", "+00:00", 1)
-	return time.Parse(time.RFC3339, s)
+	// 解析 ISO8601（含 .SSSZ 毫秒后缀，如 2026-06-20T09:33:17.000Z）
+	return time.Parse(time.RFC3339Nano, s)
 }
 
 func defaultESQuery(q string) string {