chore(host): remove unused Database:MachineDataDb option, validation, config and docs

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

@@ -7,8 +7,6 @@ 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/DatabaseOptionsValidator.cs

@@ -4,7 +4,7 @@ namespace ZB.MOM.WW.ScadaBridge.Host;
 
 /// <summary>
 /// Validates <see cref="DatabaseOptions"/> at host startup (arch-review 08
-/// round 2 NF4). All three connection settings are nullable and role-dependent
+/// round 2 NF4). Both connection settings are nullable and role-dependent
 /// (central needs the SQL Server strings, a site needs only the SQLite path),
 /// so a <c>null</c> value is valid. A present-but-whitespace value is not — it
 /// signals a mis-set config key that would fail opaquely at first DB use.
@@ -15,7 +15,6 @@ public sealed class DatabaseOptionsValidator : OptionsValidatorBase<DatabaseOpti
     protected override void Validate(ValidationBuilder builder, DatabaseOptions options)
     {
         RequireNotWhitespace(builder, options.ConfigurationDb, nameof(DatabaseOptions.ConfigurationDb));
-        RequireNotWhitespace(builder, options.MachineDataDb, nameof(DatabaseOptions.MachineDataDb));
         RequireNotWhitespace(builder, options.SiteDbPath, nameof(DatabaseOptions.SiteDbPath));
     }
 

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

@@ -73,9 +73,6 @@ 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")
                 // 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