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/Repositories/IProviderRepository.cs

@@ -0,0 +1,12 @@
+using HrynCo.NotificationService.DAL.Abstract.Providers;
+
+namespace HrynCo.NotificationService.DAL.Abstract.Repositories;
+
+public interface IProviderRepository
+{
+    Task<IReadOnlyList<Provider>> GetByServiceAsync(string serviceName, CancellationToken ct = default);
+    Task<Provider?> GetByIdAsync(Guid id, CancellationToken ct = default);
+    Task AddAsync(Provider provider, CancellationToken ct = default);
+    Task UpdateAsync(Provider provider, CancellationToken ct = default);
+    Task DeleteAsync(Provider provider, CancellationToken ct = default);
+}