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/Update/UpdateEmailChannelHandler.cs

@@ -1,4 +1,3 @@
-using HrynCo.DAL.Abstract;
 using HrynCo.NotificationService.DAL.Abstract.Repositories;
 using HrynCo.NotificationService.Services.Core;
 using HrynCo.NotificationService.Services.Logging;
@@ -10,13 +9,16 @@ internal sealed class UpdateEmailChannelHandler
     : RequestHandler<UpdateEmailChannelCommand, ServiceResult<Unit>>
 {
     private readonly IEmailChannelRepository _channels;
+    private readonly INotificationServiceRepository _services;
 
     public UpdateEmailChannelHandler(
         IContextualSerilogLogger<UpdateEmailChannelCommand> logger,
-        IEmailChannelRepository channels)
+        IEmailChannelRepository channels,
+        INotificationServiceRepository services)
         : base(logger)
     {
         _channels = channels;
+        _services = services;
     }
 
     protected override async Task<ServiceResult<Unit>> DoOnHandle(
@@ -27,6 +29,10 @@ internal sealed class UpdateEmailChannelHandler
         if (channel is null)
             return Failure<Unit>("Email channel not found.", ServiceErrorCode.NotFound);
 
+        if (request.ServiceId == Guid.Empty || await _services.GetAsync(request.ServiceId, cancellationToken) is null)
+            return Failure<Unit>("Select an existing service.", ServiceErrorCode.InvalidRequest);
+
+        channel.ServiceId = request.ServiceId;
         channel.Priority = request.Priority;
         channel.Settings = request.Settings;
         channel.DailyLimit = request.DailyLimit;