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/Entities/EmailChannelEntity.cs

@@ -0,0 +1,22 @@
+using HrynCo.NotificationService.DAL.Abstract.Entities;
+using HrynCo.NotificationService.DAL.Abstract.Providers;
+
+namespace HrynCo.NotificationService.DAL.EF.Entities;
+
+internal class EmailChannelEntity : Entity
+{
+    public required string ServiceName { get; set; }
+    public int Priority { get; set; }
+    public EmailChannelType EmailChannelType { get; set; }
+
+    /// <summary>
+    /// EmailChannel-specific credentials and settings stored as JSONB.
+    /// Deserialized based on <see cref="EmailChannelType"/> in the repository.
+    /// </summary>
+    public required string SettingsJson { get; set; }
+
+    public int? DailyLimit { get; set; }
+    public int? MonthlyLimit { get; set; }
+    public int WarnThresholdPercent { get; set; }
+    public bool IsActive { get; set; }
+}