feat(config): add ClusterNode.MaintenanceMode — the hatch the live gate proved missing

Phase 1 gate step 4 failed: setting Enabled = 0 to take a node out of service
returns 422 ClusterEnabledNodeCountMismatch, because
DraftValidator.ValidateClusterTopology requires the enabled-node count to equal
ServerCluster.NodeCount. On a Warm/Hot pair — every cluster on the rig, and
every cluster in the target topology — Enabled can therefore never be the
maintenance hatch. The plan called step 4 "the check that makes step 3
acceptable to ship", so Task 3's behaviour change was not shippable as it stood:
a node down for maintenance would block every deployment to its cluster.

The two rules were each reasonable and contradictory together — the validator
reads Enabled as "part of the declared topology", Phase 1 additionally read it
as "expect an ack". Split the meanings rather than weaken either rule:

  Enabled          part of the declared topology  (validator, untouched)
  MaintenanceMode  expected to participate now    (coordinator + reconciler)

Rejected alternatives: counting configured rather than enabled nodes (drops the
guard against booting a pair into InvalidTopology); downgrading the rule to a
warning (weakens a deploy gate for everyone); shipping with no hatch.

Sabotage: dropping !n.MaintenanceMode from the coordinator query turns the new
test red. It also asserts both nodes remain Enabled, so the fix cannot quietly
regress to disabling the row after all.

Configuration.Tests 95/95, ControlPlane.Tests 101/101, solution builds clean.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Migrations/OtOpcUaConfigDbContextModelSnapshot.cs

@@ -92,6 +92,9 @@ namespace ZB.MOM.WW.OtOpcUa.Configuration.Migrations
                     b.Property<DateTime?>("LastSeenAt")
                         .HasColumnType("datetime2(3)");
 
+                    b.Property<bool>("MaintenanceMode")
+                        .HasColumnType("bit");
+
                     b.Property<int>("OpcUaPort")
                         .HasColumnType("int");