fix(host): add MachineDataDb startup validation for Central (reverts Host-008, M2.9 #17)

REQ-HOST-3/REQ-HOST-4 require a MachineDataDb connection string for Central nodes.
The shipped docker appsettings (docker/central-node-a/appsettings.Central.json and
central-node-b) already carry the key. Host-008 had removed the fail-fast Require
because MachineDataDb had no consumer yet; this commit reverses that decision so a
misconfigured or missing connection string is caught at startup with a clear error.

Changes:
- DatabaseOptions: add MachineDataDb property with XML doc comment
- StartupValidator: add .Require for ScadaBridge:Database:MachineDataDb inside the
  existing Central .When block, immediately after the ConfigurationDb Require
- StartupValidatorTests: rename Central_MissingMachineDataDb_PassesValidation ->
  FailsValidation and flip to Assert.Throws; update comment to cite REQ-HOST-3/4,
  shipped docker appsettings, and the Host-008 reversal; add MachineDataDb to
  ValidCentralConfig() so all other Central tests remain green
- CentralDbTestEnvironment: supply ScadaBridge__Database__MachineDataDb env var
  (mirrors ConfigurationDb pattern) so HostStartupTests, HealthCheckTests, and
  MetricsEndpointTests pass through the new Require
- CompositionRootTests, AkkaHostedServiceAuditWiringTests, ActorPathTests: set
  ScadaBridge__Database__MachineDataDb env var alongside the pepper env var and
  clear it in Dispose, matching the existing pepper handling pattern

Build: 0 warnings, 0 errors. dotnet test Host.Tests: 233/233 passed.

src/ZB.MOM.WW.ScadaBridge.Host/DatabaseOptions.cs

@@ -7,6 +7,8 @@ public class DatabaseOptions
 {
     /// <summary>Connection string for the central configuration SQL Server database.</summary>
     public string? ConfigurationDb { get; set; }
+    /// <summary>Connection string for the central machine-data SQL Server database.</summary>
+    public string? MachineDataDb { get; set; }
     /// <summary>File system path to the site-local SQLite database directory.</summary>
     public string? SiteDbPath { get; set; }
 }

src/ZB.MOM.WW.ScadaBridge.Host/StartupValidator.cs

@@ -60,6 +60,9 @@ public static class StartupValidator
                 .Require("ScadaBridge:Database:ConfigurationDb",
                     _ => !string.IsNullOrEmpty(configuration.GetSection("ScadaBridge:Database")["ConfigurationDb"]),
                     "connection string required for Central")
+                .Require("ScadaBridge:Database:MachineDataDb",
+                    _ => !string.IsNullOrEmpty(configuration.GetSection("ScadaBridge:Database")["MachineDataDb"]),
+                    "connection string required for Central")
                 // Task 1.4: the LDAP server key moved into the nested Security:Ldap
                 // sub-section (bound to the shared LdapOptions). Validate the nested key so
                 // the pre-host preflight still fails fast on a missing LDAP server for