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/Providers/ProviderSettings.cs

@@ -0,0 +1,21 @@
+namespace HrynCo.NotificationService.DAL.Abstract.Providers;
+
+public abstract class ProviderSettings
+{
+    public abstract ProviderType ProviderType { get; }
+}
+
+public class SmtpProviderSettings : ProviderSettings
+{
+    public override ProviderType ProviderType => ProviderType.Smtp;
+
+    public required string Host { get; set; }
+    public int Port { get; set; } = 587;
+    public required string Username { get; set; }
+    public required string Password { get; set; }
+    public bool UseSsl { get; set; } = true;
+    public required string FromEmail { get; set; }
+    public required string FromName { get; set; }
+    public required string AppDisplayName { get; set; }
+    public required string AppBaseUrl { get; set; }
+}