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/DesignTimeDbContextFactory.cs

@@ -0,0 +1,29 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Design;
+using Microsoft.Extensions.Configuration;
+
+namespace ScadaLink.ConfigurationDatabase;
+
+/// <summary>
+/// Factory for creating DbContext instances at design time (used by dotnet ef tooling).
+/// Reads connection string from Host's appsettings.Central.json.
+/// </summary>
+public class DesignTimeDbContextFactory : IDesignTimeDbContextFactory<ScadaLinkDbContext>
+{
+    public ScadaLinkDbContext CreateDbContext(string[] args)
+    {
+        var configuration = new ConfigurationBuilder()
+            .SetBasePath(Path.Combine(Directory.GetCurrentDirectory(), "..", "ScadaLink.Host"))
+            .AddJsonFile("appsettings.json", optional: true)
+            .AddJsonFile("appsettings.Central.json", optional: true)
+            .Build();
+
+        var connectionString = configuration["ScadaLink:Database:ConfigurationDb"]
+            ?? "Server=localhost,1433;Database=ScadaLink_Config;User Id=sa;Password=YourPassword;TrustServerCertificate=True";
+
+        var optionsBuilder = new DbContextOptionsBuilder<ScadaLinkDbContext>();
+        optionsBuilder.UseSqlServer(connectionString);
+
+        return new ScadaLinkDbContext(optionsBuilder.Options);
+    }
+}