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.Worker.Services/EmailProcessing/SendEmailService.cs

@@ -9,6 +9,7 @@ using Microsoft.Extensions.Logging;
 internal sealed class SendEmailService : ISendEmailService
 {
     private readonly IEmailChannelRepository _channelRepository;
+    private readonly INotificationServiceRepository _services;
     private readonly IEmailChannelUsageRepository _usageRepository;
     private readonly IEmailTemplateService _templateService;
     private readonly IEmailTemplateRenderingService _templateRenderingService;
@@ -18,6 +19,7 @@ internal sealed class SendEmailService : ISendEmailService
 
     public SendEmailService(
         IEmailChannelRepository channelRepository,
+        INotificationServiceRepository services,
         IEmailChannelUsageRepository usageRepository,
         IEmailTemplateService templateService,
         IEmailTemplateRenderingService templateRenderingService,
@@ -26,6 +28,7 @@ internal sealed class SendEmailService : ISendEmailService
         ILogger<SendEmailService> logger)
     {
         _channelRepository = channelRepository;
+        _services = services;
         _usageRepository = usageRepository;
         _templateService = templateService;
         _templateRenderingService = templateRenderingService;
@@ -93,7 +96,9 @@ internal sealed class SendEmailService : ISendEmailService
 
     private async Task<EmailChannel> ResolveChannelAsync(string serviceName, CancellationToken ct)
     {
-        var channels = await _channelRepository.GetByServiceAsync(serviceName, ct);
+        var service = await _services.GetByNameAsync(serviceName, ct)
+            ?? throw new InvalidOperationException($"Service '{serviceName}' does not exist.");
+        var channels = await _channelRepository.GetByServiceAsync(service.Id, ct);
 
         return channels
                    .Where(c => c.IsActive)