PR 7.1 — Default-flip Galaxy backend to mxgateway

Adds Galaxy.DefaultBackend = "GalaxyMxGateway" to the server
appsettings as the forward-looking default for tooling and migration
scripts that author new Galaxy DriverInstance rows. No runtime
behavior change — both factories register independently at startup,
so existing rows keep working until PR 7.2 retires the legacy
registration (gated on the parity matrix in
docs/v2/Galaxy.ParityMatrix.md going fully green on the parity rig).

The e2e-config.sample.json comment is updated to reflect the new
default endpoint (http://localhost:5120 mxaccessgw) while still
pointing pre-flip rigs at the legacy OtOpcUaGalaxyHost path.

Install-Services.ps1's OtOpcUaGalaxyHost registration is intentionally
unchanged — yanking that mid-flight without a soaked parity rig would
leave any in-progress installation without a Galaxy backend at all.
PR 7.2 retires it alongside the legacy projects.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Server/appsettings.json

@@ -5,7 +5,22 @@
   "Node": {
     "NodeId": "node-dev-a",
     "ClusterId": "cluster-dev",
-    "ConfigDbConnectionString": "Server=localhost,14330;Database=OtOpcUaConfig;Integrated Security=True;TrustServerCertificate=True;Encrypt=False;",
+    "ConfigDbConnectionString": "Server=10.100.0.35,14330;Database=OtOpcUaConfig;Integrated Security=True;TrustServerCertificate=True;Encrypt=False;",
     "LocalCachePath": "config_cache.db"
+  },
+  "Historian": {
+    "Wonderware": {
+      "Enabled": false,
+      "PipeName": "OtOpcUaWonderwareHistorian",
+      "SharedSecret": "",
+      "PeerName": "OtOpcUa-node-dev-a",
+      "DriverInstancePrefix": "galaxy",
+      "ConnectTimeoutSeconds": 10,
+      "CallTimeoutSeconds": 30
+    }
+  },
+  "Galaxy": {
+    "$comment": "PR 7.1 — DefaultBackend selects which factory tooling and migration scripts pick when authoring NEW Galaxy DriverInstance rows. Both factories register at startup so existing rows keep working: 'GalaxyMxGateway' (default since PR 7.1, in-process .NET 10 driver over the mxaccessgw gRPC gateway) and 'Galaxy' (legacy out-of-process Galaxy.Host EXE + named pipe). Legacy registration retires in PR 7.2 once the parity matrix in docs/v2/Galaxy.ParityMatrix.md is fully green on the parity rig.",
+    "DefaultBackend": "GalaxyMxGateway"
   }
 }