Fix issue with failed background job emails not being sent (#2187)

Fixes #2186 

Background job emails will no longer fail to send for jobs unrelated to
file replication or replica deletion.

Also uses `AnyJob` for paginated background job response model, to fix
typing being out of data following addition of other types of background
jobs and lower overhead for adding new ones moving forward.

backend/btrixcloud/models.py

@@ -2354,7 +2354,7 @@ class EmptyResponse(BaseModel):
 class PaginatedBackgroundJobResponse(PaginatedResponse):
     """Response model for paginated background jobs"""
 
-    items: List[Union[CreateReplicaJob, DeleteReplicaJob]]
+    items: List[AnyJob]
 
 
 # ============================================================================

chart/email-templates/failed_bg_job

@@ -10,7 +10,18 @@ Job ID: {{ job.id }}
 Started: {{ job.started.isoformat(sep=" ", timespec="seconds") }}Z
 Finished: {{ finished.isoformat(sep=" ", timespec="seconds") }}Z
 
+{% if job.object_type %}
 Object type: {{ job.object_type }}
+{% endif %}
+
+{% if job.object_id %}
 Object ID: {{ job.object_id }}
+{% endif %}
+
+{% if job.file_path %}
 File path: {{ job.file_path }}
+{% endif %}
+
+{% if job.replica_storage %}
 Replica storage name: {{ job.replica_storage.name }}
+{% endif %}