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/PushChannelTestCredentials.cs

@@ -0,0 +1,17 @@
+using System.Security.Cryptography;
+using System.Text.Json;
+
+namespace HrynCo.NotificationService.Web.IntegrationTests;
+
+internal static class PushChannelTestCredentials
+{
+    public static string Create(string projectId = "test-project")
+    {
+        using var rsa = RSA.Create(2048);
+        return JsonSerializer.Serialize(new
+        {
+            type = "service_account", project_id = projectId,
+            client_email = "test@example.invalid", private_key = rsa.ExportPkcs8PrivateKeyPem()
+        });
+    }
+}