feat(push): add service-owned firebase channels and delivery

Add push contracts, provider delivery and channel diagnostics. Link email channels to services and document migration and validation.

Ref: IT-1074

HrynCo.NotificationService.Services/EmailChannels/Create/CreateEmailChannelHandler.cs

@@ -1,4 +1,3 @@
-using HrynCo.DAL.Abstract;
 using HrynCo.NotificationService.DAL.Abstract.Providers;
 using HrynCo.NotificationService.DAL.Abstract.Repositories;
 using HrynCo.NotificationService.Services.Core;
@@ -11,21 +10,27 @@ internal sealed class CreateEmailChannelHandler
     : RequestHandler<CreateEmailChannelCommand, ServiceResult<Guid>>
 {
     private readonly IEmailChannelRepository _channels;
+    private readonly INotificationServiceRepository _services;
 
     public CreateEmailChannelHandler(
         IContextualSerilogLogger<CreateEmailChannelCommand> logger,
-        IEmailChannelRepository channels)
+        IEmailChannelRepository channels,
+        INotificationServiceRepository services)
         : base(logger)
     {
         _channels = channels;
+        _services = services;
     }
 
     protected override async Task<ServiceResult<Guid>> DoOnHandle(
         CreateEmailChannelCommand request, CancellationToken cancellationToken)
     {
+        if (request.ServiceId == Guid.Empty || await _services.GetAsync(request.ServiceId, cancellationToken) is null)
+            return Failure<Guid>("Select an existing service.", ServiceErrorCode.InvalidRequest);
+
         var channel = new EmailChannel
         {
-            ServiceName = request.ServiceName,
+            ServiceId = request.ServiceId,
             Priority = request.Priority,
             EmailChannelType = request.ChannelType,
             Settings = request.Settings,