feat: add repository layer with IUnitOfWork and fixed EF base

- ITransaction, IUnitOfWork in DAL.Abstract
- EfTransactionAdapter, EfUnitOfWork<TDbContext>, NotificationUnitOfWork in DAL.EF
- NotificationEfRepository<TEntity>: async-only base, fixed Exists (AnyAsync),
  fixed batch Add (AddRangeAsync), single SaveChangesAsync per operation
- TemplateRepository, ProviderRepository, ProviderUsageRepository
- ProviderUsageRepository.IncrementAsync uses atomic PostgreSQL upsert
- ProviderRepository deserializes settings polymorphically via ProviderType discriminator

Ref: IT-628

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

HrynCo.NotificationService.DAL.Abstract/ITransaction.cs

@@ -0,0 +1,7 @@
+namespace HrynCo.NotificationService.DAL.Abstract;
+
+public interface ITransaction : IAsyncDisposable
+{
+    Task CommitAsync(CancellationToken cancellationToken = default);
+    Task RollbackAsync(CancellationToken cancellationToken = default);
+}

HrynCo.NotificationService.DAL.Abstract/IUnitOfWork.cs

@@ -0,0 +1,10 @@
+namespace HrynCo.NotificationService.DAL.Abstract;
+
+public interface IUnitOfWork
+{
+    Task<ITransaction> BeginTransactionAsync(CancellationToken cancellationToken = default);
+    ITransaction? GetCurrentTransaction();
+
+    Task ExecuteInTransactionAsync(Func<Task> action);
+    Task<TResult> ExecuteInTransactionAsync<TResult>(Func<Task<TResult>> action);
+}