Phase 1 WP-1: EF Core DbContext with Fluent API mappings for all 26 entities

ScadaLinkDbContext with 10 configuration classes (Fluent API only), initial
migration creating 25 tables, environment-aware migration helper (auto-apply
dev, validate-only prod), DesignTimeDbContextFactory, optimistic concurrency
on DeploymentRecord. 20 tests verify schema, CRUD, relationships, cascades.

src/ScadaLink.ConfigurationDatabase/ServiceCollectionExtensions.cs

@@ -1,12 +1,29 @@
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.DependencyInjection;
 
 namespace ScadaLink.ConfigurationDatabase;
 
 public static class ServiceCollectionExtensions
 {
+    /// <summary>
+    /// Registers the ScadaLinkDbContext with the provided SQL Server connection string.
+    /// </summary>
+    public static IServiceCollection AddConfigurationDatabase(this IServiceCollection services, string connectionString)
+    {
+        services.AddDbContext<ScadaLinkDbContext>(options =>
+            options.UseSqlServer(connectionString));
+
+        return services;
+    }
+
+    /// <summary>
+    /// Registers the ScadaLinkDbContext with no connection string (for backward compatibility / Phase 0 stubs).
+    /// This overload is a no-op placeholder; callers should migrate to the overload that accepts a connection string.
+    /// </summary>
     public static IServiceCollection AddConfigurationDatabase(this IServiceCollection services)
     {
-        // Phase 0: skeleton only
+        // Retained for backward compatibility during migration.
+        // Site nodes do not use the configuration database, so this is intentionally a no-op.
         return services;
     }
 }