refactor: rename domain types and introduce TransactionBehavior pattern

- Rename Template -> EmailTemplate, Provider -> EmailChannel,
  ProviderSettings -> EmailChannelSettings, ProviderType -> EmailChannelType,
  ProviderUsage -> EmailChannelUsage throughout all layers
- Add Undefined = 0 to EmailChannelType enum for safe default handling
- Remove SaveChangesAsync from EfRepository methods — repositories now only stage changes
- Add SaveChangesAsync to IUnitOfWork and EfUnitOfWork
- Add TransactionBehavior MediatR pipeline: wraps every handler in a transaction,
  saves and commits on success, rolls back on exception
- Add MediatR package reference to Services project

Ref: IT-628

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

HrynCo.NotificationService.DAL.EF/Repositories/EmailChannelUsageRepository.cs

@@ -0,0 +1,44 @@
+using HrynCo.NotificationService.DAL.Abstract.Repositories;
+using HrynCo.NotificationService.DAL.EF.Core;
+using HrynCo.NotificationService.DAL.EF.Entities;
+using Microsoft.EntityFrameworkCore;
+
+namespace HrynCo.NotificationService.DAL.EF.Repositories;
+
+internal sealed class EmailChannelUsageRepository : EfRepository<EmailChannelUsageEntity>, IEmailChannelUsageRepository
+{
+    public EmailChannelUsageRepository(NotificationDbContext dbContext) : base(dbContext)
+    {
+    }
+
+    public async Task<int> GetDailyCountAsync(Guid providerId, DateOnly date, CancellationToken ct = default)
+    {
+        EmailChannelUsageEntity? entity = await DbSet
+            .FirstOrDefaultAsync(x => x.ProviderId == providerId && x.Date == date, ct);
+
+        return entity?.SentCount ?? 0;
+    }
+
+    public async Task<int> GetMonthlyCountAsync(Guid providerId, int year, int month, CancellationToken ct = default)
+    {
+        return await DbSet
+            .Where(x => x.ProviderId == providerId
+                        && x.Date.Year == year
+                        && x.Date.Month == month)
+            .SumAsync(x => x.SentCount, ct);
+    }
+
+    public async Task IncrementUsageAsync(Guid providerId, DateOnly date, CancellationToken ct = default)
+    {
+        EmailChannelUsageEntity? entity = await DbSet
+            .FirstOrDefaultAsync(x => x.ProviderId == providerId && x.Date == date, ct);
+
+        if (entity is null)
+            await AddAsync(new EmailChannelUsageEntity { ProviderId = providerId, Date = date, SentCount = 1 }, ct);
+        else
+        {
+            entity.SentCount++;
+            Update(entity);
+        }
+    }
+}