attachment backend use

server/app/mailer/campaign/models.py

@@ -223,6 +223,7 @@ class AttachmentBasePathConfig(StrictModel):
 class AttachmentConfig(StrictModel):
     id: str | None = None
     label: str | None = None
+    base_path_id: str | None = None
     # Legacy UI helper. Current attachment resolution ignores this value and
     # treats direct files as plain file_filter patterns without wildcards.
     # Keep accepting it so existing drafts with {"type": ""}, "direct"