db-mars-cc: correct setArchiveContext GUC key for manual psql writes (app.user_id, not app.archive_actor_id)

db-mars-cc used GUC key app.archive_actor_id instead of app.user_id when calling setArchiveContext via manual psql for the Cami Baroni email-sync fix (2026-08-02, MARS-602 thread) -- archiveReason landed correctly but archivedBy is NULL on that archive row (append-only, unfixable retroactively). Fix the go-forward habit/helper so future manual archive-context psql writes use the correct key. Low priority, procedural not app-code.