fix: inherit EF entities from Entity base class

- TemplateEntity, ProviderEntity, ProviderUsageEntity now inherit Entity (from DAL.Abstract)
- Removes duplicated Id, Created, Updated properties from each entity

Ref: IT-628

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

HrynCo.NotificationService.DAL.EF/Core/NotificationEfRepository.cs

@@ -45,9 +45,13 @@ internal abstract class NotificationEfRepository<TEntity>
         await DbContext.SaveChangesAsync(ct);
     }
 
-    protected Task<bool> ExistsAsync(Expression<Func<TEntity, bool>> predicate, CancellationToken ct = default) =>
-        DbSet.AnyAsync(predicate, ct);
+    protected Task<bool> ExistsAsync(Expression<Func<TEntity, bool>> predicate, CancellationToken ct = default)
+    {
+        return DbSet.AnyAsync(predicate, ct);
+    }
 
-    protected Task SaveAsync(CancellationToken ct = default) =>
-        DbContext.SaveChangesAsync(ct);
-}
+    protected Task SaveAsync(CancellationToken ct = default)
+    {
+        return DbContext.SaveChangesAsync(ct);
+    }
+}