fix(server): resolve Medium code-review finding (Server-007)

Add configDbHealthy parameter to OpcUaApplicationHost; wire a
DbHealthCache (CanConnectAsync cached 10 s) in Program.cs so /healthz
reflects real config-DB reachability instead of the previous always-true
default; /healthz now returns 503 on a DB outage unless stale-config
cache is warm.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Server/ZB.MOM.WW.OtOpcUa.Server/OpcUa/OpcUaApplicationHost.cs

@@ -53,6 +53,7 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
     // don't opt into the new server-side history routing or alarm-condition state machine.
     private readonly IHistoryRouter? _historyRouter;
     private readonly AlarmConditionService? _alarmConditionService;
+    private readonly Func<bool>? _configDbHealthy;
 
     private readonly ILoggerFactory _loggerFactory;
     private readonly ILogger<OpcUaApplicationHost> _logger;
@@ -73,7 +74,13 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
         ZB.MOM.WW.OtOpcUa.Core.Abstractions.IReadable? virtualReadable = null,
         ZB.MOM.WW.OtOpcUa.Core.Abstractions.IReadable? scriptedAlarmReadable = null,
         IHistoryRouter? historyRouter = null,
-        AlarmConditionService? alarmConditionService = null)
+        AlarmConditionService? alarmConditionService = null,
+        // Server-007: optional cached probe for the configuration DB. When supplied,
+        // /healthz reflects real DB reachability and returns 503 on a DB outage (unless
+        // a stale-config cache is warm). When null, falls back to the prior always-true
+        // default so existing test construction sites and deployments without a DB probe
+        // are unaffected.
+        Func<bool>? configDbHealthy = null)
     {
         _options = options;
         _driverHost = driverHost;
@@ -89,6 +96,7 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
         _scriptedAlarmReadable = scriptedAlarmReadable;
         _historyRouter = historyRouter;
         _alarmConditionService = alarmConditionService;
+        _configDbHealthy = configDbHealthy;
         _loggerFactory = loggerFactory;
         _logger = logger;
     }
@@ -179,6 +187,7 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
             _healthHost = new HealthEndpointsHost(
                 _driverHost,
                 _loggerFactory.CreateLogger<HealthEndpointsHost>(),
+                configDbHealthy: _configDbHealthy,
                 usingStaleConfig: _staleConfigFlag is null ? null : () => _staleConfigFlag.IsStale,
                 prefix: _options.HealthEndpointsPrefix);
             _healthHost.Start();

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

@@ -197,6 +197,14 @@ if (wonderwareEnabled)
 builder.Services.AddSingleton<OpcUaApplicationHost>(sp =>
 {
     var registry = sp.GetRequiredService<DriverEquipmentContentRegistry>();
+
+    // Server-007: wire a real config-DB probe so /healthz reflects true DB reachability.
+    // The probe runs CanConnectAsync and caches the result for 10 s to avoid a DB round-trip
+    // on every /healthz poll. The Func<bool> surface on HealthEndpointsHost is synchronous;
+    // the cached value is updated on a background timer rather than blocking the HTTP handler.
+    var dbFactory = sp.GetRequiredService<IDbContextFactory<OtOpcUaConfigDbContext>>();
+    var dbHealthCache = new DbHealthCache(dbFactory);
+
     return new OpcUaApplicationHost(
         sp.GetRequiredService<OpcUaServerOptions>(),
         sp.GetRequiredService<DriverHost>(),
@@ -205,7 +213,8 @@ builder.Services.AddSingleton<OpcUaApplicationHost>(sp =>
         sp.GetRequiredService<ILogger<OpcUaApplicationHost>>(),
         equipmentContentLookup: registry.Get,
         historyRouter: sp.GetRequiredService<IHistoryRouter>(),
-        alarmConditionService: sp.GetRequiredService<AlarmConditionService>());
+        alarmConditionService: sp.GetRequiredService<AlarmConditionService>(),
+        configDbHealthy: () => dbHealthCache.IsHealthy);
 });
 builder.Services.AddHostedService<OpcUaServerService>();
 
@@ -261,3 +270,40 @@ builder.Services.AddSingleton<Phase7Composer>();
 
 var host = builder.Build();
 await host.RunAsync();
+
+// Server-007: lightweight cached config-DB health probe for /healthz.
+// Runs CanConnectAsync once and caches the result for 10 s so the /healthz
+// endpoint never blocks an HTTP handler thread on a DB round-trip while still
+// surfacing real DB reachability rather than the previous always-true default.
+internal sealed class DbHealthCache : IDisposable
+{
+    private static readonly TimeSpan CacheTtl = TimeSpan.FromSeconds(10);
+
+    private readonly IDbContextFactory<OtOpcUaConfigDbContext> _factory;
+    private readonly Timer _timer;
+    private volatile bool _isHealthy = true; // optimistic until first probe completes
+
+    public DbHealthCache(IDbContextFactory<OtOpcUaConfigDbContext> factory)
+    {
+        _factory = factory;
+        // Fire immediately, then repeat every CacheTtl.
+        _timer = new Timer(_ => _ = RefreshAsync(), null, TimeSpan.Zero, CacheTtl);
+    }
+
+    public bool IsHealthy => _isHealthy;
+
+    private async Task RefreshAsync()
+    {
+        try
+        {
+            await using var ctx = await _factory.CreateDbContextAsync().ConfigureAwait(false);
+            _isHealthy = await ctx.Database.CanConnectAsync().ConfigureAwait(false);
+        }
+        catch
+        {
+            _isHealthy = false;
+        }
+    }
+
+    public void Dispose() => _timer.Dispose();
+}