docs(historian): pluggable backends, SQL Server backend, HistoryUpdate, outbox v2, spec corrections

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

docs/Client.CLI.md

@@ -250,6 +250,15 @@ otopcua-cli historyupdate -u ... -n "..." --mode delete-at-time --time 2026-01-1
 | `--type` | `double` (default), `int64`, `bool`, `string`, `datetime` - how `--value` is parsed |
 | `--start` / `--end` | Range for `delete-raw` (both required; `--end` later than `--start`) |
 
+Expected per-item codes on the SQL Server historian backend: `insert` ⇒ `GoodEntryInserted` (repeat ⇒ `BadEntryExists`), `replace` ⇒ `GoodEntryReplaced`
+(`BadNoEntryExists` if nothing is stored at that time), `update` ⇒ either Good code, `delete-at-time` ⇒ `Good` / `BadNoEntryExists`; `delete-raw` reports one
+node-level code (`Good`, or `BadNoData` if the range held nothing). A node that is not historized, or a server on the Gateway historian backend (no
+updater, so no `HistoryWrite` access bit), answers `BadHistoryOperationUnsupported`; a `NaN`/`Infinity` double is `BadInvalidArgument`. The session's API key
+(`-U apikey -P otua_<keyId>_<secret>`) needs the `HistoryUpdate` permission — via a `Cluster`-scoped grant when `Security:Acl:Enabled` is off, or a scoped
+grant on the node when it is on; a key with only `WriteOperate` gets `BadUserAccessDenied`. See [Historian.md §HistoryUpdate](Historian.md#historyupdate) and
+[security.md](security.md#historyupdate-historian-writes). `historyread --aggregate` only knows the seven names in the table below; the SQL backend's other
+aggregates (Total, Range, Delta, PercentGood, …) need a different client.
+
 `delete-raw` is sent with `IsDeleteModified=false`. Invalid or inconsistent options (unknown mode, unpaired
 value/time, unparsable value, options that do not apply to the mode) fail before any connection is made.