fix: add missing FK migration for EmailChannelUsage -> EmailChannel

- EF model had a pending HasOne/WithMany relationship not in migrations
- Adds FK_email_channel_usage_email_channels_provider_id with cascade delete

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.DAL.EF/Migrations/NotificationDbContextModelSnapshot.cs

@@ -170,6 +170,15 @@ namespace HrynCo.NotificationService.DAL.EF.Migrations
                     b.ToTable("email_templates", (string)null);
                 });
 
+            modelBuilder.Entity("HrynCo.NotificationService.DAL.EF.Entities.EmailChannelUsageEntity", b =>
+                {
+                    b.HasOne("HrynCo.NotificationService.DAL.EF.Entities.EmailChannelEntity", null)
+                        .WithMany("UsageRecords")
+                        .HasForeignKey("ProviderId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
             modelBuilder.Entity("HrynCo.NotificationService.DAL.EF.Entities.EmailTemplateEntity", b =>
                 {
                     b.OwnsMany("HrynCo.NotificationService.DAL.EF.Entities.EmailTemplateVariableData", "Variables", b1 =>
@@ -202,6 +211,11 @@ namespace HrynCo.NotificationService.DAL.EF.Migrations
 
                     b.Navigation("Variables");
                 });
+
+            modelBuilder.Entity("HrynCo.NotificationService.DAL.EF.Entities.EmailChannelEntity", b =>
+                {
+                    b.Navigation("UsageRecords");
+                });
 #pragma warning restore 612, 618
         }
     }