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/Templates/Template.cs

@@ -0,0 +1,14 @@
+using HrynCo.NotificationService.DAL.Abstract.Entities;
+
+namespace HrynCo.NotificationService.DAL.Abstract.Templates;
+
+public class Template : Entity
+{
+    public required string ServiceName { get; set; }
+    public required string Key { get; set; }
+    public required string LanguageCode { get; set; }
+    public required string Subject { get; set; }
+    public required string HtmlBody { get; set; }
+    public required string TextBody { get; set; }
+    public IReadOnlyList<TemplateVariable> Variables { get; set; } = [];
+}

HrynCo.NotificationService.DAL.Abstract/Templates/TemplateVariable.cs

@@ -0,0 +1,7 @@
+namespace HrynCo.NotificationService.DAL.Abstract.Templates;
+
+public record TemplateVariable
+{
+    public required string Name { get; init; }
+    public bool Required { get; init; }
+}