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.Tests/DevelopmentDataSeederTests.cs

@@ -13,10 +13,16 @@ public sealed class DevelopmentDataSeederTests
     private readonly IEmailTemplateRepository _templates = Substitute.For<IEmailTemplateRepository>();
     private readonly INotificationServiceRepository _services = Substitute.For<INotificationServiceRepository>();
 
+    public DevelopmentDataSeederTests()
+    {
+        _services.SaveAsync(Arg.Any<NotificationServiceInfo>(), true, Arg.Any<CancellationToken>())
+            .Returns(call => { call.Arg<NotificationServiceInfo>().Id = Guid.NewGuid(); return Task.CompletedTask; });
+    }
+
     [Fact]
     public async Task SeedAsync_WhenConfigurationIsMissing_CreatesMailpitChannelAndTestTemplate()
     {
-        _channels.GetByServiceAsync("TestService", Arg.Any<CancellationToken>())
+        _channels.GetByServiceAsync(Arg.Any<Guid>(), Arg.Any<CancellationToken>())
             .Returns(Array.Empty<EmailChannel>());
         _templates.GetAsync("TestService", "TestEmail", "en", Arg.Any<CancellationToken>())
             .Returns((EmailTemplate?)null);
@@ -39,10 +45,10 @@ public sealed class DevelopmentDataSeederTests
     [Fact]
     public async Task SeedAsync_WhenConfigurationExists_DoesNotDuplicateOrOverwriteIt()
     {
-        _channels.GetByServiceAsync("TestService", Arg.Any<CancellationToken>())
+        _channels.GetByServiceAsync(Arg.Any<Guid>(), Arg.Any<CancellationToken>())
             .Returns([new EmailChannel
             {
-                ServiceName = "TestService",
+                ServiceId = Guid.NewGuid(),
                 EmailChannelType = EmailChannelType.Smtp,
                 Settings = new SmtpChannelSettings()
             }]);
@@ -75,7 +81,7 @@ public sealed class DevelopmentDataSeederTests
 
     private static bool IsMailpitChannel(EmailChannel channel)
     {
-        return channel.ServiceName == "TestService" &&
+        return channel.ServiceId != Guid.Empty &&
                channel.IsActive &&
                channel.Settings is SmtpChannelSettings smtp &&
                smtp.Host == "mailpit" &&