feat(host): wire UseWindowsService so sc.exe-installed service runs cleanly

The v2 plan's blessed install path (scripts/install/Install-Services.ps1)
registers the host via `sc.exe create binPath=...OtOpcUa.Host.exe`, but the
binary never called `UseWindowsService`. Without it, the Service Control
Manager waits ~30s for the process to call SetServiceStatus(Running) and
then kills it — the install script's design was incomplete.

Two changes:

- Host.csproj: drop the `IsOSPlatform('Windows')` condition on the
  Microsoft.Extensions.Hosting.WindowsServices package reference so the
  package is always available. The runtime helper used by
  UseWindowsService gates on WindowsServiceHelpers.IsWindowsService()
  internally, so it's a no-op when running as a console app or under
  Linux/macOS — the binary stays cross-platform-buildable.

- Program.cs: call builder.Host.UseWindowsService(options =>
  options.ServiceName = "OtOpcUaHost") immediately after CreateBuilder.
  When the host is launched by SCM, WindowsServiceLifetime takes over
  the IHostLifetime slot and reports START/STOP correctly. When launched
  by `dotnet run` or `OtOpcUa.Host.exe` from a console, it's a no-op.

Verified end-to-end on wonder-app-vd03.zmr.zimmer.com: `sc.exe create`
followed by `sc.exe start OtOpcUaHost` transitions from START_PENDING to
RUNNING; /login + /health/ready + /health/active all return 200; service
survives SSH session close and auto-starts on boot per the AUTO_START
flag set by the installer script.

src/Server/ZB.MOM.WW.OtOpcUa.Host/Program.cs

@@ -33,6 +33,12 @@ var builder = WebApplication.CreateBuilder(args);
 // regardless of ASPNETCORE_ENVIRONMENT.
 builder.WebHost.UseStaticWebAssets();
 
+// Windows Service support: when the EXE is started by Service Control Manager (sc.exe),
+// the host needs to call SetServiceStatus to keep the SCM happy. UseWindowsService()
+// installs the WindowsServiceLifetime IFF WindowsServiceHelpers.IsWindowsService() is
+// true at runtime — so it's safely a no-op when running as a console app or on Linux.
+builder.Host.UseWindowsService(options => options.ServiceName = "OtOpcUaHost");
+
 // Per-role appsettings overlay: appsettings.{role}.json (single role) or appsettings.admin-driver.json
 // (both). Optional — base appsettings.json carries enough to boot if these don't exist.
 var roleSuffix = roles.Length == 0 ? null : string.Join('-', roles.OrderBy(r => r, StringComparer.Ordinal));

src/Server/ZB.MOM.WW.OtOpcUa.Host/ZB.MOM.WW.OtOpcUa.Host.csproj

@@ -16,7 +16,11 @@
     <ItemGroup>
         <PackageReference Include="Akka.Hosting"/>
         <PackageReference Include="Serilog.AspNetCore"/>
-        <PackageReference Include="Microsoft.Extensions.Hosting.WindowsServices" Condition="$([MSBuild]::IsOSPlatform('Windows'))"/>
+        <!-- Always referenced (drops earlier IsOSPlatform condition) so Program.cs can
+             call builder.Host.UseWindowsService() unconditionally; the runtime helper
+             only activates when actually running as a Windows Service, so this is a
+             no-op on macOS/Linux. -->
+        <PackageReference Include="Microsoft.Extensions.Hosting.WindowsServices"/>
         <PackageReference Include="Microsoft.EntityFrameworkCore.Design">
             <PrivateAssets>all</PrivateAssets>
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>