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/NotificationConfiguration.cs

@@ -0,0 +1,66 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using ScadaLink.Commons.Entities.Notifications;
+
+namespace ScadaLink.ConfigurationDatabase.Configurations;
+
+public class NotificationListConfiguration : IEntityTypeConfiguration<NotificationList>
+{
+    public void Configure(EntityTypeBuilder<NotificationList> builder)
+    {
+        builder.HasKey(n => n.Id);
+
+        builder.Property(n => n.Name)
+            .IsRequired()
+            .HasMaxLength(200);
+
+        builder.HasMany(n => n.Recipients)
+            .WithOne()
+            .HasForeignKey(r => r.NotificationListId)
+            .OnDelete(DeleteBehavior.Cascade);
+
+        builder.HasIndex(n => n.Name).IsUnique();
+    }
+}
+
+public class NotificationRecipientConfiguration : IEntityTypeConfiguration<NotificationRecipient>
+{
+    public void Configure(EntityTypeBuilder<NotificationRecipient> builder)
+    {
+        builder.HasKey(r => r.Id);
+
+        builder.Property(r => r.Name)
+            .IsRequired()
+            .HasMaxLength(200);
+
+        builder.Property(r => r.EmailAddress)
+            .IsRequired()
+            .HasMaxLength(500);
+    }
+}
+
+public class SmtpConfigurationConfiguration : IEntityTypeConfiguration<SmtpConfiguration>
+{
+    public void Configure(EntityTypeBuilder<SmtpConfiguration> builder)
+    {
+        builder.HasKey(s => s.Id);
+
+        builder.Property(s => s.Host)
+            .IsRequired()
+            .HasMaxLength(500);
+
+        builder.Property(s => s.AuthType)
+            .IsRequired()
+            .HasMaxLength(50);
+
+        builder.Property(s => s.Credentials)
+            .HasMaxLength(4000);
+
+        builder.Property(s => s.TlsMode)
+            .HasMaxLength(50);
+
+        builder.Property(s => s.FromAddress)
+            .IsRequired()
+            .HasMaxLength(500);
+    }
+}