feat: add EF migrations and design-time factory

- Fix EmailEmailTemplateEntityConfiguration -> EmailTemplateEntityConfiguration
- Rename tables to match domain: templates->email_templates,
  providers->email_channels, provider_usage->email_channel_usage
- Add NotificationDbContextFactory for design-time migrations tooling
- Add InitialCreate migration: email_templates, email_channels, email_channel_usage

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.DAL.EF/NotificationDbContextFactory.cs

@@ -0,0 +1,16 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Design;
+
+namespace HrynCo.NotificationService.DAL.EF;
+
+internal sealed class NotificationDbContextFactory : IDesignTimeDbContextFactory<NotificationDbContext>
+{
+    public NotificationDbContext CreateDbContext(string[] args)
+    {
+        var options = new DbContextOptionsBuilder<NotificationDbContext>()
+            .UseNpgsql("Host=localhost;Port=5432;Database=notification_service;Username=postgres;Password=postgres")
+            .Options;
+
+        return new NotificationDbContext(options);
+    }
+}