Site: inject INodeIdentityProvider where NotificationSubmit is built; stamp
SourceNode = NodeName at construction.
Central: NotificationOutboxActor.HandleSubmit copies submit.SourceNode onto
the Notification row; the repository INSERT persists it (EF tracked-entity
insert flows it through automatically; raw-SQL extension if not).
After this commit, every Notifications row carries the originating site
node-a/node-b in SourceNode. Existing notifications submitted pre-feature
remain NULL.
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