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/GetByService/GetEmailChannelsHandler.cs

@@ -10,19 +10,25 @@ internal sealed class GetEmailChannelsHandler
     : RequestHandler<GetEmailChannelsQuery, ServiceResult<IReadOnlyList<EmailChannel>>>
 {
     private readonly IEmailChannelRepository _channels;
+    private readonly INotificationServiceRepository _services;
 
     public GetEmailChannelsHandler(
         IContextualSerilogLogger<GetEmailChannelsQuery> logger,
-        IEmailChannelRepository channels)
+        IEmailChannelRepository channels,
+        INotificationServiceRepository services)
         : base(logger)
     {
         _channels = channels;
+        _services = services;
     }
 
     protected override async Task<ServiceResult<IReadOnlyList<EmailChannel>>> DoOnHandle(
         GetEmailChannelsQuery request, CancellationToken cancellationToken)
     {
-        var channels = await _channels.GetByServiceAsync(request.ServiceName, cancellationToken);
+        var service = await _services.GetByNameAsync(request.ServiceName, cancellationToken);
+        if (service is null)
+            return Success<IReadOnlyList<EmailChannel>>([]);
+        var channels = await _channels.GetByServiceAsync(service.Id, cancellationToken);
         return Success(channels);
     }
 }