Práctica-create uploads to api-images before the transaction commits: pre-row orphans, no sweeper

src/app/(protected)/practicas/nueva uploads to api-images (~line 599) BEFORE the práctica row exists and before the insert transaction (715-912). Any submission that fails validation or fails inside that transaction leaves an orphaned file in api-images with nothing referencing it — no row, no sweeper. Distinct from MARS-42 (size measurement) and MARS-677 (upstream reachability). Found by coder-mars-cc 2026-09-05 while answering a terra cross-project retention question. Fix: either don't upload until the transaction commits, or add a reconciliation sweep for unreferenced api-images keys.