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/NotificationEfRepository.cs

@@ -0,0 +1,53 @@
+using System.Linq.Expressions;
+using Microsoft.EntityFrameworkCore;
+
+namespace HrynCo.NotificationService.DAL.EF.Core;
+
+internal abstract class NotificationEfRepository<TEntity>
+    where TEntity : class
+{
+    protected NotificationDbContext DbContext { get; }
+    protected DbSet<TEntity> DbSet { get; }
+
+    protected NotificationEfRepository(NotificationDbContext dbContext)
+    {
+        DbContext = dbContext;
+        DbSet = dbContext.Set<TEntity>();
+    }
+
+    protected async Task AddAsync(TEntity entity, CancellationToken ct = default)
+    {
+        await DbSet.AddAsync(entity, ct);
+        await DbContext.SaveChangesAsync(ct);
+    }
+
+    protected async Task AddRangeAsync(IEnumerable<TEntity> entities, CancellationToken ct = default)
+    {
+        await DbSet.AddRangeAsync(entities, ct);
+        await DbContext.SaveChangesAsync(ct);
+    }
+
+    protected async Task UpdateAsync(TEntity entity, CancellationToken ct = default)
+    {
+        DbSet.Update(entity);
+        await DbContext.SaveChangesAsync(ct);
+    }
+
+    protected async Task DeleteAsync(TEntity entity, CancellationToken ct = default)
+    {
+        DbSet.Remove(entity);
+        await DbContext.SaveChangesAsync(ct);
+    }
+
+    protected async Task DeleteRangeAsync(IEnumerable<TEntity> entities, CancellationToken ct = default)
+    {
+        DbSet.RemoveRange(entities);
+        await DbContext.SaveChangesAsync(ct);
+    }
+
+    protected Task<bool> ExistsAsync(Expression<Func<TEntity, bool>> predicate, CancellationToken ct = default) =>
+        DbSet.AnyAsync(predicate, ct);
+
+    protected Task SaveAsync(CancellationToken ct = default) =>
+        DbContext.SaveChangesAsync(ct);
+}