fix(vision): reference image sopravvive al copy-on-write (#38.6)

_copy_tasks_to_version non copiava le righe VisionReferenceImage del
task sorgente: il task copiato perdeva le immagini di riferimento del
grafo di visione. Ora copia le righe sul task nuovo, stesso path e
stessi metadati (stesso file su disco, non duplicato).
Di conseguenza il file va cancellato solo se nessun'altra riga
VisionReferenceImage referenzia lo stesso path: nuova
vision_service.file_still_referenced(), usata dalla route DELETE
prima di remove_reference_file.
Test: copy-on-write elenca l'immagine sulla nuova versione; DELETE di
una delle due righe lascia il file; DELETE dell'ultima lo rimuove.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/services/vision_service.py

@@ -275,6 +275,22 @@ def remove_reference_file(reference: VisionReferenceImage) -> None:
     _reference_file_path(reference).unlink(missing_ok=True)
 
 
+async def file_still_referenced(db: AsyncSession, path: str) -> bool:
+    """True if some VisionReferenceImage row still points at *path*.
+
+    Copy-on-write (#38.6) can leave several rows sharing the same file on
+    disk (the copy carries over the source's `path` rather than duplicating
+    the file). The router must call this, after the row being deleted has
+    been committed, before it calls `remove_reference_file` - otherwise
+    deleting one task's reference would take the file out from under every
+    other task that still references it.
+    """
+    result = await db.execute(
+        select(VisionReferenceImage.id).where(VisionReferenceImage.path == path).limit(1)
+    )
+    return result.scalar_one_or_none() is not None
+
+
 async def save_reference_image(
     db: AsyncSession,
     task_id: int,