fix(historian): #668 review follow-ups (explicit modified source time, paging tests, ACL test, QuestDB orphan warning, docs)

- HistoryModifiedValue gains explicit SourceTimeUtc (SqlServer + QuestDB populate it); the node manager's resume cursor uses it instead of the prior value's nullable source timestamp (MinValue for inserts).
- ACL test now uses a trail-returning data source and asserts Good + data on the allow leg.
- Node-manager tests: end-only backward open-ended paging, reverse resume cursor, null-prior cursor.
- QuestDB updater logs a Warning (tag + time range) when a sample write fails after its trail rows landed.
- docs/Historian.md: rolling-upgrade window, IDENTITY-vs-commit ordering, QuestDB cross-node ModifiedId clock ordering.

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs

@@ -6218,7 +6218,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
             {
                 var last = page.Values[^1];
                 outboundCp = _historyContinuationStore.Save(session, new HistoryContinuationState(
-                    tagname, last.Value.SourceTimestampUtc ?? DateTime.MinValue, endUtc, 0, numValuesPerNode,
+                    tagname, last.SourceTimeUtc, endUtc, 0, numValuesPerNode,
                     HistoryCursorKind.Modified, last.ModifiedId, startUtc));
             }