feat: add base entity abstractions and domain model to DAL.Abstract

- IEntity<TId>, Entity<TId>, Entity base classes in Entities/
- Template, TemplateVariable domain models in Templates/
- Provider, ProviderSettings, SmtpProviderSettings, ProviderUsage, ProviderType in Providers/
- ITemplateRepository, IProviderRepository, IProviderUsageRepository in Repositories/
- ProviderUsage tracks daily counts; monthly derived by summing daily records
- IEntity<TId> lives in DAL.Abstract (not DAL.EF) — boundary enforced from the start

Ref: IT-628

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

HrynCo.NotificationService.DAL.Abstract/Entities/Entity.cs

@@ -0,0 +1,17 @@
+namespace HrynCo.NotificationService.DAL.Abstract.Entities;
+
+public abstract class Entity<TId> : IEntity<TId> where TId : struct
+{
+    public TId Id { get; set; }
+    public DateTimeOffset Created { get; set; }
+    public DateTimeOffset? Updated { get; set; }
+}
+
+public abstract class Entity : Entity<Guid>
+{
+    protected Entity()
+    {
+        Id = Guid.NewGuid();
+        Created = DateTimeOffset.UtcNow;
+    }
+}