fix: run heavy reconstruction in threadpool + lock so the health probe stays alive during 1-2min jobs; return 500 on server errors

Fix broadcast crash: grid >= largest piece; crop-safe paste/place

Guard stray-drop None (no crash on few/non-A4 pieces)

Add guarded background-remover (real-photo front-end; no-op on dataset)

Deploy Forensic DocStitch API (geometry solver + verdict)

initial commit