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.EF/Core/EfTransactionAdapter.cs

@@ -0,0 +1,23 @@
+using HrynCo.NotificationService.DAL.Abstract;
+using Microsoft.EntityFrameworkCore.Storage;
+
+namespace HrynCo.NotificationService.DAL.EF.Core;
+
+internal sealed class EfTransactionAdapter : ITransaction
+{
+    private readonly IDbContextTransaction _transaction;
+
+    internal EfTransactionAdapter(IDbContextTransaction transaction)
+    {
+        _transaction = transaction;
+    }
+
+    public Task CommitAsync(CancellationToken cancellationToken = default) =>
+        _transaction.CommitAsync(cancellationToken);
+
+    public Task RollbackAsync(CancellationToken cancellationToken = default) =>
+        _transaction.RollbackAsync(cancellationToken);
+
+    public ValueTask DisposeAsync() =>
+        _transaction.DisposeAsync();
+}