feat(galaxy): operator-authenticated alarm ack — send Galaxy credential to mxaccessgw (#749)

mxaccessgw 488649c+ acknowledges by AuthenticateUser as the operator and
writing <alarm>.AckMsg, so AcknowledgeAlarmRequest now needs a real Galaxy
operator_user + operator_domain + operator_password. The OPC UA data plane is
API-key/anonymous and carries no Galaxy password, so the driver resolves the
account per ack:

1. the session's API key's Galaxy ack identity, set on AdminUI /apikeys and
   held ONLY in the encrypted Secrets store
   (otopcua/apikeys/<keyId>/galaxy-ack-{user,password}); a key with a user but
   no password secret is refused, never silently downgraded;
2. else the driver instance's AlarmAck fallback account (DriverConfig
   FallbackUser/FallbackDomain/FallbackPasswordSecretRef; secret:/${secret:}/
   env:/file:/dev: only — a literal is refused);
3. else DriverOperationRefusedException: nothing is sent, never an empty
   operator_user. Refusals are excluded from the resilience retry/breaker.

- Pins ZB.MOM.WW.MxGateway.Client/Contracts 0.2.1 -> 0.3.0 (vendored into
  nuget-packages/ from the Gitea feed, per the local-mxgw source mapping).
- The session ApiKeyId (not a secret) rides NativeAlarmAck ->
  RouteNativeAlarmAck -> RouteAlarmAck -> AlarmAcknowledgeRequest; the password
  is resolved in the driver and never travels through actor messages.
- The OPC UA principal + key id are appended to the AckMsg comment
  ("[OPC UA: <principal>, key <id>]") and logged, so a shared fallback
  account does not lose who acknowledged.
- Password never logged: GalaxyAckCredential.ToString omits it, the
  acknowledger scrubs it from every gateway message it echoes and withholds
  any exception that echoes it; audit rows carry only the key id.
- AdminUI: /apikeys "Ack identity" editor (set/remove; delete-key clears it),
  Galaxy driver form gains the fallback section.
- Synchronous OPC UA status for a refusal is follow-up #760 (the native ack
  path is fire-and-forget after the Part 9 method returns).

Tests: request mapping (password populated), password absent from logs and
exception text (mutation-checked), key-mapped / fallback / refused paths at
resolver and driver level, breaker non-trip, key-id propagation through node
manager and DriverHostActor, AdminUI service + form round-trip.

Closes #749

docs/security.md

@@ -345,6 +345,14 @@ of a credential was wrong.
 ConfigDb, so keys reach them inside the deployment artifact exactly as `NodeAcl` rows do. Clearing
 `Enabled` takes effect on a node only once a new artifact reaches it.
 
+**Galaxy alarm-ack identity per key (lmxopcua#749).** mxaccessgw acknowledges Galaxy alarms as a real
+Galaxy account, so a key can carry an optional Galaxy ack identity (AdminUI `/apikeys` → "Ack
+identity"). It is held only in the encrypted Secrets store
+(`otopcua/apikeys/<keyId>/galaxy-ack-{user,password}`), never in the ConfigDb or the artifact; keys
+without one and anonymous sessions use the Galaxy driver's `AlarmAck` fallback account, and with
+neither the ack is refused before it reaches the gateway. Deleting a key on `/apikeys` removes its
+identity. See [docs/drivers/Galaxy.md § Alarm acknowledge credential](drivers/Galaxy.md#alarm-acknowledge-credential).
+
 ### Anonymous is a normal principal
 
 Anonymous sessions are **admitted by default** (`OpcUa:AllowAnonymous`, default `true` — flipping the