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.Migrator/DevelopmentDataSeeder.cs

@@ -32,8 +32,8 @@ internal sealed class DevelopmentDataSeeder
         if (string.IsNullOrWhiteSpace(serviceName))
             throw new InvalidOperationException("Development seed service name is not configured.");
 
-        await SeedMailpitChannelAsync(serviceName, cancellationToken);
-        await SeedServiceAsync(serviceName, cancellationToken);
+        var serviceId = await SeedServiceAsync(serviceName, cancellationToken);
+        await SeedMailpitChannelAsync(serviceId, serviceName, cancellationToken);
 
         foreach (EmailTemplate template in CreateTemplates(serviceName))
         {
@@ -55,22 +55,23 @@ internal sealed class DevelopmentDataSeeder
         }
     }
 
-    private async Task SeedServiceAsync(string serviceName, CancellationToken ct)
+    private async Task<Guid> SeedServiceAsync(string serviceName, CancellationToken ct)
     {
         var services = await _services.GetAllAsync(ct);
         var service = services.SingleOrDefault(s => s.Name == serviceName);
         if (service?.Languages.Any(l => l.Code == DefaultLanguageCode) == true)
-            return;
+            return service.Id;
         var isNew = service is null;
         service ??= new NotificationServiceInfo { Name = serviceName };
         service.Languages = [..service.Languages, new ServiceLanguageInfo { Code = DefaultLanguageCode }];
         await _services.SaveAsync(service, isNew, ct);
+        return service.Id;
     }
 
-    private async Task SeedMailpitChannelAsync(string serviceName, CancellationToken cancellationToken)
+    private async Task SeedMailpitChannelAsync(Guid serviceId, string serviceName, CancellationToken cancellationToken)
     {
         IReadOnlyList<EmailChannel> existingChannels = await _channels.GetByServiceAsync(
-            serviceName, cancellationToken);
+            serviceId, cancellationToken);
 
         if (existingChannels.Count > 0)
         {
@@ -82,7 +83,7 @@ internal sealed class DevelopmentDataSeeder
 
         var channel = new EmailChannel
         {
-            ServiceName = serviceName,
+            ServiceId = serviceId,
             Priority = 1,
             EmailChannelType = EmailChannelType.Smtp,
             Settings = new SmtpChannelSettings