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/Configurations/SiteConfiguration.cs

@@ -0,0 +1,68 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using ScadaLink.Commons.Entities.Sites;
+
+namespace ScadaLink.ConfigurationDatabase.Configurations;
+
+public class SiteConfiguration : IEntityTypeConfiguration<Site>
+{
+    public void Configure(EntityTypeBuilder<Site> builder)
+    {
+        builder.HasKey(s => s.Id);
+
+        builder.Property(s => s.Name)
+            .IsRequired()
+            .HasMaxLength(200);
+
+        builder.Property(s => s.SiteIdentifier)
+            .IsRequired()
+            .HasMaxLength(100);
+
+        builder.Property(s => s.Description)
+            .HasMaxLength(2000);
+
+        builder.HasIndex(s => s.Name).IsUnique();
+        builder.HasIndex(s => s.SiteIdentifier).IsUnique();
+    }
+}
+
+public class DataConnectionConfiguration : IEntityTypeConfiguration<DataConnection>
+{
+    public void Configure(EntityTypeBuilder<DataConnection> builder)
+    {
+        builder.HasKey(d => d.Id);
+
+        builder.Property(d => d.Name)
+            .IsRequired()
+            .HasMaxLength(200);
+
+        builder.Property(d => d.Protocol)
+            .IsRequired()
+            .HasMaxLength(50);
+
+        builder.Property(d => d.Configuration)
+            .HasMaxLength(4000);
+
+        builder.HasIndex(d => d.Name).IsUnique();
+    }
+}
+
+public class SiteDataConnectionAssignmentConfiguration : IEntityTypeConfiguration<SiteDataConnectionAssignment>
+{
+    public void Configure(EntityTypeBuilder<SiteDataConnectionAssignment> builder)
+    {
+        builder.HasKey(a => a.Id);
+
+        builder.HasOne<Site>()
+            .WithMany()
+            .HasForeignKey(a => a.SiteId)
+            .OnDelete(DeleteBehavior.Cascade);
+
+        builder.HasOne<DataConnection>()
+            .WithMany()
+            .HasForeignKey(a => a.DataConnectionId)
+            .OnDelete(DeleteBehavior.Cascade);
+
+        builder.HasIndex(a => new { a.SiteId, a.DataConnectionId }).IsUnique();
+    }
+}