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.Web.IntegrationTests/AdminSmtpTestControllerTests.cs

@@ -89,7 +89,7 @@ public sealed class AdminSmtpTestControllerTests
         recorder.Channel = new EmailChannel
         {
             Id = ChannelId,
-            ServiceName = "TestService",
+            ServiceId = Guid.NewGuid(),
             EmailChannelType = EmailChannelType.Smtp,
             Settings = new SmtpChannelSettings
             {
@@ -97,7 +97,7 @@ public sealed class AdminSmtpTestControllerTests
                 Username = "test-user", Password = "saved-test-secret", UseSsl = true
             }
         };
-        return (new AdminChannelsController(mediator), recorder);
+        return (new AdminChannelsController(mediator, null!), recorder);
     }
 
     private static TestSmtpRequest Request() => new(