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.

tests/ScadaLink.Host.Tests/HostStartupTests.cs

@@ -43,6 +43,7 @@ public class HostStartupTests : IDisposable
                 .WithWebHostBuilder(builder =>
                 {
                     builder.UseSetting("ScadaLink:Node:Role", "Central");
+                    builder.UseSetting("ScadaLink:Database:SkipMigrations", "true");
                 });
             _disposables.Add(factory);