Resolve all 33 medium-severity architecture review findings

Second slice of the 2026-09-09 whole-system review remediation
(docs/plans/2026-09-09-arch-review.md §10), on top of the High slice 146a1323.

Runtime/devices: SR-2 alarm on-trigger integration scope; SR-3 correlated NativeAlarmActor
subscribe retry; SR-4 immutable audited DB target; SR-5 ordered InstanceActor stop/flush;
SR-12 rehydration fence; SR-13 S&F gate recheck per delivery; DC-4 no OPC UA security
downgrade; DC-5 alarm snapshot for every new subscriber; ORCH-1 sync-dispose joins the pump.

Management/UI: MG-2 site scope on audit tree + Flow Debugger Replay; MG-3 stale-probe memo
removed; MG-4 server-owned bulk-deploy deadline; UI-5 sandbox runs in a killable child
process (new CentralUI.SandboxWorker project); UI-14 bounded sandbox output/IPC.

Edge/audit: EI-1 relay audit ownership; EI-2 SkipBodyCapture suppresses error text; EI-3/EI-11
error-field redaction + caps; EI-4 single relay deadline, pre-execution-only retry; EI-5
authenticated relay provenance; EI-10 body cap before audit read.

Persistence/contracts: DL-1 global Instance.UniqueName unique index + conflict translation;
DL-2 DataConnection protocol config encrypted at rest (migration
20260909203046_EnforceGlobalInstanceNamesAndProtectProtocolConfiguration — coordinated
central upgrade, old binaries cannot read protected rows); DL-3 bounded Replay paging;
DL-6 cross-site Overwrite of a deployed instance rejected; XC-4 proto descriptor contract test.

Config/hygiene: XC-1 central listener isolation by local port; XC-2/XC-3 inert SMTP + cron
keys removed from rig settings; CM-2 env2 bootstrap guard enabled; DC-6 deploy site exit code;
DC-7 notifier retries only explicit standby rejection; XC-5 ClusterClient purged from docs.

Gates recorded in the report: build 0/0, 9 397 passed / 0 failed, rig redeployed on all
eight nodes with the migration applied. 46 Low findings remain open.

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Migrations/ScadaBridgeDbContextModelSnapshot.cs

@@ -585,9 +585,11 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
 
                     b.HasIndex("AreaId");
 
+                    b.HasIndex("SiteId");
+
                     b.HasIndex("TemplateId");
 
-                    b.HasIndex("SiteId", "UniqueName")
+                    b.HasIndex("UniqueName")
                         .IsUnique();
 
                     b.ToTable("Instances");
@@ -1335,8 +1337,7 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
                     SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
 
                     b.Property<string>("BackupConfiguration")
-                        .HasMaxLength(4000)
-                        .HasColumnType("nvarchar(4000)");
+                        .HasColumnType("nvarchar(max)");
 
                     b.Property<int>("FailoverRetryCount")
                         .ValueGeneratedOnAdd()
@@ -1349,8 +1350,7 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
                         .HasColumnType("nvarchar(200)");
 
                     b.Property<string>("PrimaryConfiguration")
-                        .HasMaxLength(4000)
-                        .HasColumnType("nvarchar(4000)");
+                        .HasColumnType("nvarchar(max)");
 
                     b.Property<string>("Protocol")
                         .IsRequired()