48 tests covering the XOR validation, the AssetItem.status/active-assignment preconditions, seat overbooking, seat/status sync across assign+return, FK-not-found for all three FKs, double-return, 404s, auth-gating, malformed bodies, and two explicit concurrency tests (Promise.all against a 1-remaining-seat License and against a single in_stock AssetItem) exercising the transaction race-closing logic in src/lib/assignments.ts.